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US intelligence agencies believe "with varying degrees of confidence" that Syria has used chemical weapons against rebels, the White House has said, according to BBC. 

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.

Speaking to reporters in Abu Dhabi on Thursday, US Defence Secretary Chuck Hagel told reporters in Abu Dhabi that the use of sarin "violates every convention of warfare."