US intelligence has found evidence that sarin gas, a chemical weapon, has been used in Syria on a small scale, the White House has said .

However, in a letter to Congress the administration made it clear that it did not believe that the evidence was conclusive, saying it only had "varying amounts of confidence" in its reliability. Nor did the evidence prove beyond any doubt that the Syrian government had been responsible for using sarin, though this was "very likely" to be the case.

Later, the US secretary of state, John Kerry, said that chemical weapons were believed to have been used in two separate attacks.

If the evidence was confirmed, the White House warned, "the United States and the international community have a number of responses available, and no option is off the table".

"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," the White House letter to Senators John McCain and Carl Levin said. "This assessment is based in part on physiological samples. Our standard of evidence must build on these intelligence assessments as we seek to establish credible and corroborated facts."

The letter said that the "chain of custody" by which the samples were thought to have made their way from Syria to the west, was "not clear, so we cannot confirm how the exposure occurred and under what conditions".

"We do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime. Thus far, we believe that the Assad regime maintains custody of these weapons, and has demonstrated a willingness to escalate its horrific use of violence against the Syrian people" the letter said. It was signed by Miguel Rodriguez, an assistant to the president charged with managing relations with Congress.

Chuck Hagel said "it [using a chemical weapon] violates every convention of warfare". The US defence secretary, in Abu Dhabi while on a tour of the Middle East, added that the US administration had reached its conclusion over the past 24 hours. "As I've said, this is serious business – we need all the facts," he said.

Senator McCain, who has long advocated US-led military intervention in Syria, said the letter showed that the 'red line' had been crossed. The wording of the letter suggested the administration was not ready to make that assessment. It did however bring US assessments more in line with UK, French and Israeli claims after several days of mixed messages from the western allies. The careful use of language and the phrase "varying degrees of confidence" suggests that there remain disagreements among the various US intelligence agencies over the strength of the evidence and that the administration was seeking to keep its options open on whether and how to respond.

Barack Obama has said the Assad's regime use of chemical weapons would be a "game changer" that could trigger a US military response, and Wednesday's letter restated that position.

"However, precisely because the president takes this issue so seriously, we have an obligation to fully investigate any and all evidence of chemical weapons use within Syria," the White House letter said, repeating western calls for a UN investigation to "credibly evaluate the evidence and establish what took place".

For the time being however, American and European officials appear ready to wait for the results of a UN inquiry into claims and counter-claims by both the Syrian opposition and the government that the other had used such weapons. UN investigators are due to visit survivors of alleged chemical weapon attacks in refugee camps, and analyse soil sample taken by western intelligence agencies.

US officials had been disparaging on Wednesday about the evidence for the use of chemical weapons in the Syrian conflict. British government officials had insisted that the intelligence agencies of both countries shared the same analysis of the evidence, but that it had taken longer for it to filter through to official pronouncements from Washington.

"There was just a longer lag in the US, but as far as I know we are absolutely on the same page on this," a British official said.

A Foreign Office spokesperson said: "We have limited but persuasive information from various sources showing chemical weapon use in Syria, including sarin. This is extremely concerning. Use of chemical weapons is a war crime. We have briefed our allies, partners and the UN on this information and we are working actively to get more and better information.

"Assad must cooperate with the international community and prove that his regime has not committed this horrific crime, allowing unrestricted access for the UN and OPCW to investigate on the ground in Syria."

British and French officials believe that Assad's forces have calibrated their use of sarin and possibly other chemical weapons to try to demoralise his opponents while stopping short of triggering a full-scale western response.

"This is a public relations tightrope for the White House," said Aram Nerguizian of the Center for Strategic and International Studies in Washington. "They are working hard to try to craft a message but it's an unenviable position to be in. The administration has to clarify what it means by 'red lines'. It really depends on the quality of the intelligence... I am sceptical that this is enough to jump on. You could set fire to the Syrian facilities with specialist ordinance designed to destroy chemical weapons but you still wouldn't affect the course of the war."