Obama Renews Warning that Syria Chemical Arms Use Will Be 'Game Changer'

U.S. President Barack Obama on Friday promised a "vigorous investigation" into reports Syrian forces fired chemical weapons and renewed his warning that proof of their use would be a "game changer."

Obama delivered the warning during talks at the White House with King Abdullah II of Jordan, as he faced rising political pressure for a military intervention in the vicious Syrian civil war.

He told reporters that U.S. authorities had "some evidence that chemical weapons have been used on the population in Syria, these are preliminary assessments, they're based on our intelligence gathering."

"We have varying degrees of confidence about the actual use, there's a range of questions about how, when, where these weapons have been used," he said.

The president said that Washington would pursue a "very vigorous investigation and would work with its partners towards a definitive answer on the chemical weapons issues as soon as possible.

He said that as horrific as it was that civilians face mortar fire and other attacks, the use of chemical weapons "crosses another line."

"That is going to be a game changer ... we have to make assessments deliberately but I think all of us, not just the United States, but around the world, have to recognize how we cannot stand by and permit the systematic of weapons like chemical weapons on civilian populations.

"This is going to be something that we're going to be paying a lot of attention to, trying to confirm, mobilize the international community around those issues."

Earlier on Friday, the White House said Obama is awaiting a "definitive judgment" on whether the Syrian regime used chemical weapons against rebel fighters before taking action.

The statement from Washington came as the Syrian opposition urged the U.N. Security Council to take immediate steps, possibly even by imposing a no-fly zone, following the U.S. announcement that the weapons had likely been used.

British Prime Minister David Cameron added his voice to the fray, saying growing evidence that Syrian President Bashar Assad had turned an arsenal of chemical agents on his own people was "extremely serious."

In Washington, where U.S. spy agencies issued the assessment that Syria had likely used chemical arms on a "small scale," the White House remained cautious.

"We are working to establish credible and corroborated facts to build on this intelligence assessment in order to establish a definitive judgment as to whether or not the president's red line has been crossed," spokesman Jay Carney said.

The spokesman said those facts would "inform our decision-making about next steps," adding that "all options remain on the table."

"The president wants the facts. And I'm not going to set a timeline, because the facts need to be what drives this investigation -- not a deadline," Carney said, shortly before Obama was set to sit down with Jordan's King Abdullah II.

"The situation in Syria is and has been grave. The Assad regime has the blood of its own people on its hands."

Carney reiterated that Washington backed a United Nations investigation into the use of chemical weapons in Syria. Damascus has asked for a U.N. probe but has since refused to let a U.N. team into the country.

Assad's government only wants its claims that opposition rebels used chemical arms to be investigated. U.N. Secretary General Ban Ki-moon has said the team should also look into opposition claims.

Syria's opposition urged the U.N. Security Council to act without delay.

"Should it find the regime used such weapons, it must act immediately, at least by imposing a no-fly zone," an official from the main opposition National Coalition told Agence France Presse on condition of anonymity.

During the 2011 uprising that ousted long-time Libyan strongman Moammar Gadhafi, the Security Council imposed a no-fly zone to prevent civilians being hit by regime air strikes.

But the Council has been deadlocked over Syria for more than two years, with permanent members Russia and China backing Assad and vetoing several draft resolutions that would have threatened sanctions on the regime.

The opposition National Coalition has accused the regime of using chemical weapons in the northern province of Aleppo, in Homs in the center and in rebel-held areas near Damascus.

Britain's Times newspaper published a report on Friday detailing the killing in Aleppo of a family, allegedly by chemical arms.

The family "died twitching, hallucinating and choking on white froth that poured from their noses and mouths. Their doctors believe that they were killed by nerve gas," said the report.

In London, Cameron -- whose Foreign Office confirmed it also had "limited but persuasive" evidence of the use of chemical agents -- said the international response would likely be political rather than military.

"This is extremely serious. And I think what President Obama said was absolutely right, that this should form for the international community a red line for us to do more," Cameron told the BBC.

"In my view what we need to do... is shape that opposition, work with them, train them, mentor them, help them so we put the pressure on the regime and so we can bring this to an end," he said.

The European Union was cautious, reiterating a request to Damascus to allow a U.N. chemical weapons probe in Syria.

Israel meanwhile led a push for military action.

"From the moment the international community understands that red lines have been crossed and that chemical weapons have been used, they will realize there's no other choice than to react (militarily)," said Deputy Foreign Minister Zeev Elkin.

Experts say containing the threat of chemical weapons would likely involve a presence of troops on the ground.

David Reeths, director of IHS Jane's Consulting, said containment operations "are extremely complex and would almost certainly require a significant in-country presence for an extended period of time."