The top two Democratic and Republican senators on national security issued a new bipartisan plea for President Barack Obama to lead a military campaign against Syria, including missile strikes and arming opposition rebels, in pointed floor speeches.

Sen. Carl Levin, D-Mich., chairman of the Senate Armed Services Committee, and Sen. John McCain, R-Ariz., took the floor with a long list of limited U.S. military interjections into the conflict. They argued such moves were not only possible, but necessary to save lives and prevent spillover instability in the Middle East.

The two men wrote Obama in March calling for intervention, but Thursday's speeches come with specific military options and far more bipartisan support on Capitol Hill for a stronger response to Bashar al-Assad's regime, especially since the White House's acknowledgement in late April that the intelligence community believes chemical weapons have been used by Syria.

"No one should think that the United States has to act alone, put boots on the ground, or destroy every Syrian air defense system to make a difference for the better in Syria," McCain said. "We have more limited options at our disposal -- including limited military options -- that can make a positive impact on this crisis."

McCain said the United States should at least target Syria's ballistic missiles, preventing the possibility that they could be fitted to carry chemical weapons.

"We could use our precision strike capabilities to target Assad's aircraft and SCUD missile launchers on the ground without our pilots having to fly into the teeth of Syria's air defenses. Similar weapons could be used to selectively destroy artillery pieces and make Assad's forces think twice about remaining at their posts. We could also use Patriot missile batteries outside of Syria to help protect safe zones inside of Syria from Assad's aerial bombing and missile attacks. 

"Would any of these options immediately end the conflict? Probably not. But they could save innocent lives in Syria," said McCain.

Levin said the Armed Services Committee next week is scheduled to receive a classified briefing it requested from top Pentagon brass on military options for Syria. Under Secretary of Defense Jim Miller, DOD's policy chief, and Lt. Gen. Terry Wolff, the J-5, or director of strategic plans and policy for the Joint Staff, are slated to appear.  The committee  will, he said, "urge them to carry a message back to the administration that it is time to up the military pressure on Assad."

But the chairman did not wait to give his own suggestion.

"In my view, the facts on the ground today make the consequences of inaction too great. It is time for the United States and our allies to use ways to alter the course of events in Syria by increasing the military pressure on Assad," Levin said.

Levin asked Obama to support Turkey in creating a "safe zone" inside Syria, deploying Patriot batteries close to the border, "neutralize" any Syria threats, and arm "vetted" opposition rebels.

He also argued that military action would give Secretary of State John Kerry political "backing" to bring Russia into a solution to end the conflict, while continuing to condemn Russia for supporting Assad's regime.

Sen. Bob Menendez, chairman of the Senate Foreign Relations Committee, who on Monday introduced a bill to arm the rebels, said the threat of a humanitarian crisis in Jordan -- which has seen half a million registered Syrian refugees cross its border, already -- was reason enough for prompt action. King Abdullah II of Jordan told senators during his April visit, Menendez said, that the population of Jordan already has increased by 20 percent due to Syrian inflows and he fears it could double.

"We cannot afford for that ally to ultimately find itself in a position in which it could very well collapse," Menendez said.

"I would suggest a bipartisan consensus is forming in the United State Senate that now is time to do more, not less, when it comes to Syria" said Sen. Lindsey Graham, R-S.C., including arming "the right rebels, the right opposition, with the right weapons."

Graham said there is now enough bipartisan support to "turn the tide in Washington," calling the trend a "monumental sea change."

"To the opposition: this is a great day for you. To Assad: this seals your fate," Graham said.

Graham also tried to connect the Syrian conflict to the specter of Islamist-inspired bombings in the United States.

"There is enough chemical weapons in Syria to kill thousands if not millions of Americans and people who are our allies," said Graham, noting that he's worried Syrian chemical weapons could end up being used inside the United States.

"The next bomb that goes off in America may have more than nails and glass in it."

The U.S. Army's top officer said that force readiness is "degrading significantly" enough that if President Barack Obama decides to put boots on the ground in Syria, soldiers may not be fully prepared for the job if they don't move out by the end of this summer.

Gen. Ray Odierno, Army chief of staff, said he believes the Free Syrian Army will prevail because the rebels have been able to win and hold territory.

"I kind of believe its not a matter of if, it's a matter of when," Odierno said of the FSA's chances to topple Syrian President Bashar al-Assad.

From a military perspective, Odierno did not offer advice to the rebels, which the U.S. support officially and with non-lethal aid, but said he was encouraged by what they've been able to accomplish against Syria's forces.

"I think from what I've seen is they have made some significant gains. I think they are controlling the territory. It makes you think that, you know, it's going to be difficult for the regime over time to survive," Odierno said, at a Defense Writers Group briefing with reporters in Washington, on Tuesday.

Odierno cautioned, however, that training cutbacks due to the sequester mean that, within months, the Army will be less prepared for a ground intervention.

"Its a matter of us having the dollars to make sure they are ready and trained to meet such a contingency in Syria."

"Readiness is OK right now, but it's degrading significantly because our training is reducing. So, the next three, four months, we probably have the capability to do it," he said, of a Syrian incursion. "Next year, it becomes a little bit more risky."

"If you ask me today, we have forces that can go. I think it will change over time because the longer we go cancelling training and reducing our training, the readiness levels go down."

When readiness falls, he warned, risk inevitably goes up.

"What is the risk? The risk is lives."

Odeirno said that while Pentagon continues to present President Obama options for Syria, including acting unilaterally or in an international coalition, he's more concerned about the future facing the region.

"What I worry about is the next day. So, when it happens, what happens the day after?" he said. "To me, that's important, what happens to Syria."

"Nothing happens independently in the Middle East. What's the impact on Israel? What's the impact on Lebanon? What's the impact on Jordan," he said, mentioning Iraq, Turkey, Saudi Arabia, and Iran, as well.

"If we don't get this right, what happens the day after ... could change the whole face of the Middle East, or it could go smoothly," he speculated. "How do we from an international coalition try to make this happen in such a way where we don't create incredible instability once Syria falls. That's what I worry about."

Add the use of chemical and biological weapons to his list of concerns.

"There's lots to worry about," Odierno said. "For me, it gets very, very messy."

There's one more layer to the Syria mess that complicates any outcome, said the Army chief: terrorists in the rebel ranks. 

"With the rebels, we do know there's some terrorists in there," Odierno said. "Obviously, we don't want them to be involved in the outcome, we don't want them to gain power because of the impact they could have on the rest of the region -- regionally and then potentially internationally."

British Defense Secretary Philip Hammond said that Britain backs President Barack Obama's insistence on "conclusive" evidence that Syria used chemical weapons before moving forward with any military intervention.

The defense leader said on Thursday that Britain's physiological samples, which led them to declare that they believed sarin gas had been used in Syria, was "compelling but not conclusive" and did not meet courtroom-worthy standards required to muster international support for a military response.

"For that evidence to have any chance of being admitted in court, it would need to have been collected under controlled conditions, secured through a documented chain of custody to the point where it was tested," Hammond told a small group of defense reporters at the British Embassy in Washington. "We do not yet have samples that meet that standard of evidence."

The unknown chain of custody for that evidence, Hammond said, may impede a quick response. But while the evidence continues to be analyzed, Britain was pressing to build international legal support and authority for a military intervention, if needed. Short of that, London is already seeking legal alternatives to acting without United Nations approval, he said.

The purity of the evidence that Bashar al-Assad's forces employed chemical weapons is particularly important to convincing Russia to end their support for the regime and support a U.N. Security Council authorization of the use of force. It is also important for Arab allies in the region that likely would be called upon to join an international intervention.

For the British public, however, Hammond said the Iraq war looms large, fueling a nationwide skeptical streak about the prospect of going to war on inconclusive intelligence.

"We in the U.K. are particularly sensitive to this. There is a strong sense in U.K. public opinion that we went to war in Iraq on the back of evidence that proved not to be correct -- what in British political space is called ‘the dodgy dossier,'" he said, of the Bush administration's claim of Iraq's imminent use of weapons of mass destruction.

Hammond said he does not know what evidence the United States may possess in addition to what Britain has shared, but that he would discuss it with U.S. Defense Secretary Chuck Hagel at the Pentagon, later on Thursday.

"I don't think there is a gap between us," Hammond said of the evidence. "I think we're now in a place where we both recognize that there is compelling but not conclusive evidence that chemical weapons have been used."

"We need hard evidence. The kind of evidence that would be admissible in court," about what, when, where, and by whom, said Hammond. "We don't yet have evidence that would pass our own legal systems tests."

In the meantime, Britain will continue to pressure the Syrian regime to allow in U.N. inspectors to establish "definitively" if chemical weapons had been used. [The team, however, has a goal to determine if chemicals were used, not necessarily the custody chain.

Hammond showed no movement toward providing direct lethal aid to the Syrian rebels.

"No decision to provide lethal aid has yet been made," he said. No options are off the table, either, but Britain is giving all it can, he said.

"We are currently providing support to the [rebels] right up to the maximum level that we can without crossing the line into lethal aid." Britain most recently provided armored 4x4 vehicles, body armor, and night vision goggles. 

Hammond also defended Obama's "red line" over the use of chemical weapons and the international community's right to deliberate over a larger military intervention in Syria. An estimated 70,000 Syrians already have died in conventional fighting.  

"Use of chemical weapons is specifically illegal under international law. It is a war crime, and therefore it takes the level of culpability of the regime to a new level. It crosses a new threshold."

If the U.N. is unable to pass a resolution, Britain is preparing other legal frameworks to act on its own, if needed.

"We are exploring other options under British interpretation of international law, under the doctrine of humanitarian necessity, which may allow us scope to act," he said. That unilateral determination was Britain's justification for military action in Bosnia, he said.

Hammond reiterated that due to sensitivities of "the neighborhood" and Russia, Middle Eastern countries must take a role in any military response, as well as in helping crafting a political future for a Syrian opposition that the West can support. 

White House officials notified Congress today that the U.S. believes Syria used sarin gas on its own people, crossing President Obama's "red line" in a potentially game changing acknowledgement that could draw the U.S. into the Syrian civil war.

"It violates every convention of warfare," Defense Secretary Chuck Hagel said, in making the announcement public on Thursday, while visiting the United Arab Emirates. 

The announcement comes days after an Israeli general during Hagel's visit earlier this week declared that Syria had used chemical weapons, which followed similar declarations from Britain and France. Hagel and U.S. officials, put on the spot, previously said U.S. intelligence was inconclusive.

"As I have said, the intelligence community has been assessing information for some time on this issue, and the decision to reach this conclusion was made in the past 24 hours," Hagel said, "and I have been in contact with senior officials in Washington today and most recently the last couple of hours on this issue."

"We cannot confirm the origin of these weapons, but we do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime."

Miguel Rodriguez, director of the Office of Legislative Affairs, in a letter to Sen. John McCain, R-Ariz., wrote, "Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin."

Rodriguez cautioned that President Obama would require further investigation of the evidence before deciding how to react, but said the U.S. remains "prepared for all contingencies" to respond to any "confirmed" use of chemical weapons.

A senior administration official briefing reporters in the afternoon would not say what President Obama would do, if the U.S. confirms Syria's use of chemical weapons to its own higher "standard of evidence."

"I don't want to get into those hypotheticals at this juncture," the official said.

But Rodriguez, in the letter to Congress, wrote, "No option is off the table."

"However, precisely because the president takes this issue so seriously, we have an obligation to fully investigate any and all evidence of chemical weapons use within Syria," he wrote. "Given the stakes involved, and what we have learned form our own recent experience, intelligence assessments alone are not sufficient -- only credible corroborated facts that provide us with some degree of certainty will guide our decision-making, and strengthen our leadership of the international community."

The White House said the assessment was made from "physiological samples," but cautioned "we cannot confirm how the exposure occurred and under what conditons."

On Wednesday, McCain sent a letter to Obama demanding a straight, and public, answer on Syria's chemical weapons use, co-signed by Sens. Carl Levin, D-Mich., Bob Corker, R-Tenn., Bob Menendez, D-N.J., Saxby Chambliss, R-Ga, Bob Casey, D-Penn., Lindsey Graham, R-S.C., and Kelly Ayotte, R-N.H.