President Assad's regime appears to have used the nerve agent sarin against Syrian rebels, UK prime minister David Cameron said as he disclosed that he would travel to Russia for urgent talks with Vladimir Putin on resolving the crisis.
Mr Cameron told British MPs there was a "growing body of limited but persuasive information" showing that the regime had used chemical weapons during the two-year civil war.
He said yesterday he would fly to Russia tomorrow for talks with Mr Putin aimed at forcing a "political transition" in Syria.
It came as Britain launched a push to have the EU's arms embargo on the rebels lifted, saying doing so would strengthen the moderate opposition and ensure the EU could respond to any chemical weapons attack.
"I can tell the House today that there is a growing body of limited but persuasive information showing that the regime has used and continues to use chemical weapons, including sarin," Mr Cameron said.
"I've tasked the chairman of the joint intelligence committee to give the national security council constantly updated assessments of the information we and allies have."
On tomorrow's talks in Russia, he added: "There's an urgent need to start a proper negotiation to force a political transition and to bring this conflict to an end, and I will be flying to Sochi on Friday to meet with President Putin to discuss this issue further."
John Kerry, the US secretary of state, called Mr Cameron to discuss a joint US and Russian initiative for an international conference on Syria later this month.
Mr Kerry said on Tuesday that the meeting would be held on the basis of an agreement reached but never implemented in Geneva last year.
The Geneva Accord proposed an interim government of both regime officials and rebel leaders to bring the civil war to a close.
Britain is also using its chemical weapons intelligence to argue that the EU arms embargo must be amended to allow the supply of weapons to opposition rebels for the defence of civilians. In a confidential paper, seen by a British newspaper, diplomats set out two "options" allowing Britain and France to start supplying arms to the official Syrian opposition by amending the embargo when it comes up for renewal at the end of the month.
"The situation in Syria is deteriorating sharply. With the likely use of chemical weapons and the growth of extremism, the conflict has entered an even more dangerous phase," the paper argues.
"We must consider all the options, (including) the ability to give further assistance to the moderate Syrian opposition. It will also protect civilians, and save lives. Crucially, it will ensure we can respond flexibly to a major escalation in the conflict, such as chemical weapons attacks."
The proposal will be discussed by foreign ministers at a meeting in Brussels on May 27.
Separately, it has emerged that at least 15 troops in Syrian President Bashar al-Assad's elite Republican Guard were killed in Israel's bombardment of the Mount Qassyoun military facility outside of Damascus in the early hours of Sunday morning.
Dozens more are still unaccounted for. (© Daily Telegraph, London)