Syria used sarin on rebels at least twice, says Cameron Link to video: Syria: Cameron responds to US chemical weapons claim David Cameron has provided fresh detail from the joint intelligence committee setting out the credible evidence the UK has gathered showing that the Assad regime in Syria has used the "abhorrent agent sarin" to attack the opposition at least twice. "There is credible evidence of multiple attacks using chemical weapons in Syria, including the use of the abhorrent agent sarin," he said. Tests undertaken at Britain's weapons research centre, Porton Down, showed sarin was used by regime forces at Utaybah on 19 March and at Sheikh Maqsood on 13 April, he said. We believe the scale of the use is sanctioned and ordered by the Assad regime. We have not seen any credible reporting of chemical weapons use by the Syrian opposition. However, we assess that elements affiliated to al-Qaida in the region have attempted to acquire chemical weapons for use in Syria. That is the picture described to me by the joint intelligence committee and I always choose my words on this subject very carefully because of the issues that there have been in the past. We have made no decision to arm the opposition but it was right to lift the arms embargo. The information about chemical weapons further shows the folly of having an embargo giving some sort of moral equivalence to President Assad and the legitimate opposition. We shall continue to train, support and assist the opposition. He said: "If we do not engage with elements of the opposition and encourage those who have a positive, democratic and pluralistic view about the future of Syria, we won't be able to influence the shape of that opposition. There is a brutal dictator who is using chemical weapons under our noses in a conflict where 100,000 people have died. Cameron said he still wanted to see a peace conference setting up a peaceful transition to a new regime. His comments came just hours after Russia dismissed US assertions that Bashar al-Assad has used chemical weapons against his own people, and said any US move to arm Syrian rebels would jeopardise efforts to convene a peace conference. Responding to White House moves to broaden its military support for the forces lined up against Assad's regime, the Kremlin said it was not convinced by the pretext for doing so. Yuri Ushakov, foreign policy adviser to President Vladimir Putin, said US officials had briefed Russia on the allegations against Assad. But I will say frankly that what was presented to us by the Americans does not look convincing. It would be hard even to call them facts. The White House said late on Thursday that it would supply direct military aid to Syria's rebels after concluding that government forces had used chemical weapons, something President Barack Obama has called a "red line." Cameron told the Guardian on Friday that Britain shared the Americans' "candid assessment." In Damascus, Syrian officials denounced the US verdict as a "caravan of lies" and said Washington's decision to arm the rebels was a "flagrant double standard" in its dealings with terrorism. "The White House ... relied on fabricated information in order to hold the Syrian government responsible for using these weapons, despite a series of statements that confirmed that terrorist groups in Syria have chemical weapons," the foreign ministry said. Russia has consistently obstructed US-led attempts to bring sanctions against Assad's regime for the bloody conflict that has resulted in almost 100,000 deaths in Syria, and the displacement of millions of people. Russian officials say they do not support Assad and are merely against foreign intervention on principle, but Moscow has continued to supply arms and other aid to Assad as its last major ally alongside Iran. The issue of arming the rebels has taken on extra urgency in recent days as pro-Assad forces are believed to be moving towards Aleppo, Syria's second city, for a possible showdown with rebel forces that could change the course of the two-year conflict. Heavy fighting was reported in Aleppo on Friday morning. It remains unclear exactly what weaponry the Americans might supply. Senator John McCain, one of the strongest proponents of US military action in Syria, said he was told on Thursday that Obama had decided to "provide arms to the rebels." Officials told the Associated Press that details were being finalised but that the weapons might include small arms, ammunition, assault rifles and a variety of anti-tank weaponry such as shoulder-fired remote-propelled grenades and other missiles.