BBC News reports: US intelligence agencies believe “with varying degrees of confidence” that Syria has used chemical weapons against rebels, the White House has said.

It said the nerve agent sarin had been deployed on a “small scale”, and did not say where or when it had been used.

The White House has warned chemical weapons use would be a “red line” for possible intervention, but says this intelligence does not represent proof.

Republicans in Congress called on Thursday for a strong US response.

The assessment was made in letters to lawmakers on Thursday signed by Miguel Rodriguez, White House director of the office of legislative affairs.

“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,” one of the letters said.

But it added: “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.”

The phrase “varying degrees of confidence” is normally used to reflect differences in opinion within the intelligence community.

The Washington Post adds: In informing Congress Thursday that Syria’s government may have used chemical agents against the population, the Obama administration stated that “no option is off the table” should future evidence confirm the mounting suspicions.

The phrase, evoking America’s current confrontation with Iran and past ones with Iraq, prompts more questions than it offers clarity for how President Obama will navigate a worsening civil war that already has killed more than 70,000 people.

Would Obama send American forces to Syria if a United Nations investigation proves Syria’s President Bashar al-Assad has used the nerve agent Sarin in restive towns? And if he would not do so at a time when, in his words, “the tide of war is receding” after more than a dozen years of overseas conflict, would U.S. prestige suffer in the eyes of allies and antagonists alike?

The administration is already behind France, Britain and Israel in asserting that Assad most likely used chemical weapons against his people, and members of Congress from both parties were quick Thursday to seize on the acknowledgment as a “game changer” for U.S. policy. Even public opinion, according to recent polling, suggests that may be the case.

“The administration has confirmed that the Assad regime in Syria has crossed a dangerous, game-changing red line,” House Minority Leader Eric Cantor (R-Va.) said in a statement, which called on members to attend a classified briefing Friday morning.

The administration has made clear that it has monitored closely allegations of the Syrian government’s chemical weapons use since December, when reports first surfaced. But Obama has sought to downplay them as much as possible, given the consequences facing the administration if true.

On Thursday, Miguel Rodriguez, Obama’s chief liaison to Congress, made clear in a letter to the Hill that the administration will continue to seek a United Nations investigation to determine definitely whether chemical weapons have been used and to what extent.

Doing so buys the administration some time to decide a course of action, even as congressional Republicans raised concern about delays.