US suspects use of chemical weapons in Syria

The US has said that Syria has likely used chemical weapons against rebel forces on a "small scale," but emphasised intelligence services were still not 100 percent sure.

US spy agencies have investigated reports from Syrian opposition groups that President Bashar al-Assad's forces have used sarin gas on at least two occasions during the two-year-old conflict.

"Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria," Caitlin Hayden, a US National Security Council spokesperson, said on Thursday.

The assessment, which she said was based in part on "physiological samples", points to the possible use of sarin, a man-made nerve agent used in two attacks in Japan in the 1990s. It can cause convulsions, respiratory failure and death.

Hayden however warned the chain of custody of the weapons was "not clear, so we cannot confirm how the exposure occurred and under what conditions".

President Barack Obama has declared that the deployment of chemical weapons would be a game-changer and has threatened unspecified consequences if it happened. Even so, the Obama administration will likely move carefully, mindful of the lessons of the start of the Iraq war more than a decade ago.

At the time, the US administration used inaccurate US intelligence to justify the invasion of Iraq in pursuit of nuclear, chemical and biological weapons that turned out not to exist.

Also on Thursday, Britain said it has "limited but persuasive" evidence of the use of chemical weapons in Syria, including sarin gas.

"We have limited but persuasive information from various sources showing chemical weapon use in Syria, including sarin," a Foreign Office spokesperson said.

"This is extremely concerning. Use of chemical weapons is a war crime," she added.

"We have briefed our allies, partners and the UN on this information and we are working actively to get more and better information."

Reports earlier this month said British military scientists had studied a soil sample brought back from an area close to Damascus and found it tested positive for chemical weapons, although the government has not confirmed that.

Meanwhile, UN leader Ban Ki-moon renewed a call for Syria to let inspectors into the country.

Syria asked for a UN investigation but has since refused to let a UN team waiting in the region into the country.

Assad's government only wants its claims that opposition rebels used chemical arms to be investigated. Ban has said the team should also look into opposition claims.

Ban has noted the US claims and "takes seriously the assessment presented," UN spokesman Martin Nesirky said, commenting on the comments from the US.

"However, the United Nations is not in a position to comment on assessments based on national intelligence information."