There is a "possibility" that Russia has tampered with evidence of an alleged chemical weapons attack in Douma, the trigger for US, UK and French air strikes in Syria, the British ambassador to the UN said.

Syrian President Bashar al-Assad has said claims he was behind a deadly gas attack targeting his own people is "100 percent fabrication".

Syria's foreign minister has categorically denied using chemical gasses to kill the country's own civilians this week.

Update11.18am: Three civilians - a mother and two children - have died in a suspected chlorine gas attack on an opposition-held district in the city of Aleppo, a Syrian rescue worker said.