Britain has echoed the United States, saying there is "limited but persuasive" evidence that chemical weapons have been used in Syria's bloody civil war.
Downing Street did not say whether it believed the regime or the rebels had been responsible for their use, but the US said on Thursday it believed "with some degree of varying confidence" that the Syrian government had used chemical weapons.
Defence Secretary Chuck Hagel said it was suspected that sarin gas had been used against the rebels.
"It violates every convention of warfare," he said.
President Barack Obama has previously said the use of chemical weapons would be a "game-changer" in relation to US intervention in the Syria.
However a senior defence official cautioned that the White House letter was not an "automatic trigger" for a decision on the use of military force.
The British Foreign Office said there was "limited but persuasive evidence" of chemical weapon use, including sarin.
It said: "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 (Organisation for the Prohibition of Chemical Weapons) to investigate on the ground in Syria.”
Both London and Washington called on Mr Assad to allow United Nations inspectors to conduct a full investigation to establish what had happened.
There was a clear determination in both capitals that - after the experience of Iraq - they should not be drawn into precipitate action on the basis of intelligence which could turn out to be faulty.
No information was made public on what quantity of chemical weapons might have been used, or when or what casualties might have resulted.
But it is understood that samples smuggled out of Syria were analysed by the Defence Science and Technology Laboratory at Porton Down, Wiltshire.
The White House has previously said the use of chemical weapons would be a "game-changer" in terms of intervention in the bloody conflict.
US Secretary of State John Kerry said later there were two cases of chemical weapons being used which the US had been made aware of.
It was not immediately clear what quantity of weapons might have been used, or when or what casualties might have resulted.
The revelations come after Tuesday's announcement byIsrael, who claimed Syria had already used chemical weapons.
In a letter to US senators, White House legislative director Miguel Rodriguez said: "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."
While Rodriguez reiterated that the use of chemical weapons remained a "red line" issue for the US, he also indicated the would have to be a full investigation of the evidence.
"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," he said.
There was some impatience among US politicians, who have been pressing the President to take a tougher line on Syria.
Speaking to reporters, Republican Senator John McCain called for a "safe zone" to be set up for the rebels to operate from.
Sarin gas was used by Saddam Hussein against the Kurdish town Halabja in 1988, leaving thousands dead.
The nerve agent was also used in a terrorist attack that killed 13 people in Tokyo's subway system in 1995.
There has been no official confirmation that such weapons have been used under President Bashar Assad's regime, though in March a US intelligence official said there was a "high probability" they were.