Russia may have visited the site of an alleged poison gas attack in the Syrian town of Douma and “tampered with” evidence, the US ambassador to the global chemical arms watchdog said Monday.

“It is our understanding the Russians may have visited the attack site,” US ambassador Ken Ward told emergency talks of the Organisation for the Prohibition of Chemical Weapons (OPCW) in The Hague.

“We are concerned they may have tampered with it with the intent of thwarting the efforts of the OPCW fact-finding mission to conduct an effective investigation,” he added in his speech, a copy of which was sent to AFP.

If proven this would raise “serious questions” about the ability of the fact-finding mission to do its job, he added.

The OPCW’s executive council was meeting behind closed doors Monday to discuss the April 7 attack on Douma in which 40 people were said to have been killed by suspected chlorine gas.

Ward called on the 41-member governing body of the OPCW “to condemn the Syrian government for its reign of chemical terror and demand international accountability for those responsible for these heinous attacks.”

He also urged Syria to “end this charade,” “immediately cease all” such attacks and “immediately declare and dismantle all aspects of its chemical weapons program.”