There is "limited but growing" evidence that Syrian government troops have used chemical weapons, UK Prime Minister David Cameron says.
"It is extremely serious, this is a war crime," Mr Cameron told the BBC.
On Thursday, the White House said that US intelligence agencies believed "with varying degrees of confidence" that Syria had used the nerve agent sarin on a "small scale".
Syrian officials have denounced the allegations as "lies".
Opposition activists and state media meanwhile report fierce fighting between government troops and rebels in a number of suburbs of the capital, Damascus.
Mr Cameron said he agreed with the White House's warning that chemical weapons use would be a "red line" for possible intervention.
However, the US 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.
No details were given of where or when sarin had been used.
The letter 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, US Defence Secretary Chuck Hagel said the use of sarin "violates every convention of warfare".
The UK Foreign Office echoed the US claims, saying it had "limited but persuasive information from various sources" of chemical weapons use in Syria.
It is understood that Britain obtained samples from inside Syria that have been tested by the Defence Science and Technology Laboratory at Porton Down, Wiltshire.
"Material from inside Syria tested positive for sarin," a Foreign Office spokesman said.
On Friday, Syrian official Sharif Shehadeh told the Associated Press the US allegations were "lies", saying that similar US accusations about Iraq's possession of weapons of mass destruction had proved untrue.
Syria's Deputy Foreign Minister Faisal Mekdad also dismissed the accusations in an earlier interview with Reuters,
Syria is believed to possess large quantities of chemical weapons and there has been heightened concern among the international community in recent months about the safety of the stockpiles.
Although there have been numerous accusations, there has so far not been any confirmation that chemical weapons have been used during Syria's two-year-old conflict.
Anthony Loyd, a journalist for the Times newspaper, told the BBC about the aftermath of one suspected chemical weapons attack earlier this month in the northern city of Aleppo.
Video shown to him by doctors treating the affected patients "showed pretty clearly that they had been gassed", Mr Loyd says.
None of the patients appeared to have been hit by shrapnel but were frothing at the mouth, had dilated pupils and several other symptoms suggesting the use of chemical weapons, he added.
BBC world affairs correspondent, Nick Childs, says the use of chemical weapons has long been perceived as especially horrific because they are seen as particularly inhumane and indiscriminate, not least in the wake of public revulsion over their deployment during World War I, which led to efforts to outlaw them.
US President Barack Obama warned in December that Syrian President Bashar al-Assad would face "consequences" if he used such weapons.
The letters released on Thursday were sent to powerful US senators John McCain and Carl Levin.
In response, Senator McCain told reporters a "red line has been crossed" and recommended arming the opposition, a step the White House has been reluctant to take.
High-profile Democratic lawmakers also called for action to help secure Syria's stockpile of chemical arms and increase aid to the opposition, including the possible imposition of a no-fly zone.
On Friday Israeli Deputy Foreign Minister Zev Elkin hinted that the US should consider military action to "take control" of Syria's chemical weapons.
"It is clear that if the United States wants to and the international community wants to, they could act - inter alia, militarily... And then all the fears... will not be relevant," Mr Elkin told Israeli radio.
Mr Cameron said he was "keen for us to do more" in helping opposition forces in Syria.
"We want our allies and partners to do more with us to shape that opposition to make sure we're supporting people with good motives," he said.
Meanwhile, opposition activists reported fierce fighting in the Barzeh district of northern Damascus on Friday, saying that the army and pro-government militiamen had pushed into the area backed by tank fire.
The state-run Sana news agency said troops had killed a number of rebels in fighting in the Jobar and Zamalka districts of the capital.
According to the UN, at least 70,000 people have been killed in the Syrian conflict.
Syria's government and rebels have accused each other of using chemical weapons. A UN team is trying to enter Syria to investigate.