The United States on Thursday accused Russia of helping Syria conceal the use of banned toxic munitions in the civil war by undermining the work of the global chemical weapons agency trying to identify those responsible.

The comments by the US representative to the Organisation for the Prohibition of Chemical Weapons (OPCW), Kenneth Ward, drew a rapid denial from Moscow and came as Western powers and Russia clashed at the agency’s annual conference in The Hague.

Moscow has for months cited dissent by two former OPCW employees who leaked a document and an email as evidence that the OPCW doctored the conclusions of a March 1 report which found that a toxic chemical containing chlorine was used in a 2018 attack near Damascus.

More than 40 people were killed in that attack in Douma, a town on the outskirts of the capital then held by rebels, on April 7, 2018.

The United States, Britain and France retaliated a week later by firing missiles at Syrian government targets, the biggest Western military action against the Damascus authorities of the eight-year-old war.

Syria and Russia deny there ever was a chemical attack in Douma, saying the event was staged using bodies brought from elsewhere, and that the OPCW’s report on Douma was doctored to justify Western military intervention.