US intelligence agencies believe Syria's government has likely used chemical weapons on a small scale, the White House said on Thursday, but added that President Barack Obama needed "credible and corroborated" facts before acting on that assessment.
The disclosure of the assessment, which Defense Secretary Chuck Hagel said was made within the past 24 hours and the White House said was based in part on physiological samples, triggered immediate calls for US action by members of Congress who advocate deeper US involvement.
But while President Barack Obama declared that Syrian use of chemical weapons would be a game-changer, his administration made clear it would move carefully - mindful of the lessons of the start of the Iraq war 10 years ago.
Then, the George W. Bush administration used faulty intelligence to justify the Iraq invasion in pursuit of nuclear, chemical and biological weapons that turned out not to exist.
"Given the stakes involved, and what we have learned from our own recent experiences, intelligence assessments alone are not sufficient - only credible and corroborated facts that provide us with some degree of certainty will guide our decision-making," Miguel Rodriguez, White House director of the office of legislative affairs, said in a letter to lawmakers.
One senior US defense official told reporters that "we have seen very bad movies before" where intelligence was perceived to have driven policy decisions that later, in the cold light of day, were proven wrong.
The White House said the US intelligence community assessed with varying degrees of confidence that the chemical agent sarin was used by the Syrian government. But it noted that "the chain of custody is not clear."
"So we cannot confirm how the exposure occurred and under what conditions," according to the White House letter, sent to lawmakers.
The term "varying degrees of confidence" also usually suggested some debate within the intelligence community about the assessment, the defense official noted.
The scale of the sarin use appeared limited, with one US intelligence official noting that nobody was "seeing any mass casualties" from any Syrian chemical weapons use.
France, Britain and Israel have concluded evidence suggests chemical arms have been used in Syria's conflict.
A top Israeli military intelligence officer said on Tuesday that evidence supported the conclusion Syrian government forces had used chemical weapons - probably sarin - several times against rebels trying to overthrow President Bashar al-Assad.
Red line crossed?
Republican Senator John McCain of Arizona, one of the leading advocates of deeper U.S. involvement in Syria's civil war, said the US intelligence assessment demanded Washington follow with action.
"The president of the United States said that if Bashar Assad used chemical weapons, it would be a game changer, that it would cross a red line," he said.
"I think it's pretty obvious that red line has been crossed."
Senator Bob Corker, the top Republican on the Senate Foreign Relations Committee, said there was likely still a need to check on chemical weapons use.
"There realistically is probably some additional steps that need to be taken to verify, but ... there are indications a red line has been crossed," he told reporters.
Syria's deputy foreign minister, Faisal Mekdad, told Reuters that US aid to the rebels may backfire and lead to attacks on American soil like those of Sept. 11, 2001.
"Once the fire of terrorism spreads in Syria it will go everywhere in the world," he said in an interview in Damascus.