WASHINGTON (Reuters) – 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.

The Prime Minister’s Office had no comment on Hagel’s statement that chemical weapons were used in Syria.

The defense secretary’s admission came two days after Brig.-Gen. Itai Brun, head of Military Intelligence’s Research Division, said, “We believe the regime has, and is using, chemical weapons” – probably sarin – against rebels trying to overthrow Assad, and that it has done so several times.

The Prime Minister’s Office also refused to relate to those comments, apparently out of a desire to avoid a conflict with the US administration over the matter. Obama has said that Assad’s use of chemical weapons would constitute a “red line” that would trigger an American response.

But while Obama declared that Syrian use of chemical weapons would be a gamechanger, 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 assessmentscredible 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 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 that evidence suggests chemical arms have been used in Syria’s conflict.

Republican Sen. John McCain of Arizona, one of the leading advocates of deeper US 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.”

Sen. Bob Corker of Tennessee, 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,” Corker told reporters.

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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 September 11, 2001.

“Once the fire of terrorism spreads in Syria it will go everywhere in the world,” he said in an interview in Damascus.

The White House has not specified what action Obama might take if he determines with certainty that Syria has used chemical weapons. But in its letter to lawmakers, it said it was “prepared for all contingencies.”

Herb Keinon contributed to this report.

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