There is "limited but growing" evidence that Syrian government troops have used chemical weapons, UK Prime Minister David Cameron said.

"It is extremely serious, this is a war crime," Cameron told the BBC.

On Thursday, April 25, the White House said that U.S. intelligence agencies believed "with varying degrees of confidence" that Syria had used the nerve agent sarin. It said the gas had been deployed on a "small scale", but did not give details of where or when it had been used.

Cameron said he agreed with the White House's warning that chemical weapons use would be a "red line" for possible intervention. However, the U.S. has said that this latest intelligence does not represent proof of chemical weapons use.

The White House's 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, U.S. Defense Secretary Chuck Hagel said the use of sarin "violates every convention of warfare".

The UK Foreign Office echoed the U.S. claims, saying it had "limited but persuasive information from various sources" of chemical weapons use in Syria, the BBC reported.