John Kerry says Assad's Syria regime HAS used sarin chemical weapons against rebels - despite Barack Obama insisting that was a 'red line' for US

Chuck Hagel and John Kerry reveal nerve gas has been used – but they will not invade for now

US Secretary of Defense Chuck Hagel speaks with reporters after reading a statement on chemical weapon use in Syria (AP)

John Kerry has said that chemical weapons have been used by the Assad regime against rebel forces in Syria, breaching what President Barack Obama has previously described as a “red line” for America.

The US Secretary of State’s assessment followed a letter sent by Chuck Hagel, America’s new Defence Secretary, to Republican Senator John McCain in which Mr Hagel said that with “some degree of varying confidence” the White House had determined that chemical weapons – and specifically Sarin gas – had been deployed.

The letter represents a change of stance from Mr Hagel, who on Tuesday rejected claims by Israel’s head of military intelligence, Itai Brun, that the Syrian government had used chemical weapons on several occasions. Mr Hagel was writing to Mr McCain and another Senator, Democrat Carl Levin, the chairman of the Senate Committee on Armed Services, who had requested information about Mr Hagel’s earlier comments. Mr Hagel told reporters in Abu Dhabi last night that “any use of chemical weapons in Syria very likely originated” from the Syrian government. His words will heap further pressure on the West to intervene in the two-year-old civil war, which has already cost 70,000 lives and displaced more than a million people. Other than the provision of non-lethal equipment, Nato countries have not heeded calls inside Syria to prevent further bloodshed with military action.

The pressure will be most acute for Mr Obama, who last August said that the discovery of chemical weapons use by the Syrian military would be a “game-changer” for the US. “We have been very clear to the Assad regime but also to other players on the ground that a red line for us is [when] we start seeing a whole bunch of weapons moving around or being utilised,” he said. “That would change my calculus. That would change my equation.”

But Mr Hagel’s letter stressed that the new information was being treated with caution. With echoes of the failed intelligence that led to the invasion of Iraq in 2003, he continued: “Given the stakes involved, and what we have learned from our experience, intelligence assessments alone are not sufficient – only credible and corroborated facts that provide us with some degree of certainty will guide our decisions.”

Mr McCain, who has advocated military action in Syria, said the letter was designed to give the White House “an out”. He told Fox News: “I worry that the President... will use these caveats as an excuse not to act right away, or to act at all.”

Britain and France last month asked the UN to investigate chemical weapons use in Damascus on 19 March and Homs on 23 December. The Syrian regime has maintained that it would not turn such weapons on its own people and that any use had been by rebel fighters. The Foreign Office last night said: “We have limited but persuasive information... showing chemical weapon use in Syria, including Sarin. This is extremely concerning. Use of chemical weapons is a war crime.”

‘We need to be very, very careful’

US intelligence analysts were almost certainly provided with blood and hair samples by rebel sources while examining whether President Assad’s forces used chemical weapons, according to diplomatic sources. But tests have not yielded conclusive proof, despite indications that it was Sarin.

The letter sent by the White House to members of Congress confirming the Obama administration’s belief that chemical weapons were used stressed: “Our standard of evidence must build on these intelligence assessments as we seek to establish credible and corroborated facts. For example, the chain of custody is not clear, so we cannot confirm how the exposure occurred.”

A senior American diplomatic source said: “We need to be very, very careful about this, we all remember what happened with Iraq. This is not just a rush to prove whether a ‘red line’ has been crossed or not, it’s a matter of getting it right. We would much rather the UN also carries out its own tests.“

Kim Sengupta