Chemical weapons experts are due to arrive in Douma on Wednesday to probe an alleged poison gas attack, Russian officials said, as the US voiced fears Moscow may already have “tampered with” evidence at the site.

Following weekend missile strikes on Syria by the US, France and Britain, Russia traded accusations with Western nations on Monday, dismissing as “a blatant lie” accusations that Moscow was hindering the investigation in Douma.

The four nations confronted each other in tense emergency talks at the Organisation for the Prohibition of Chemical Weapons (OPCW) in The Hague as inspectors prepare for a difficult and dangerous mission.

Syrian state media early on Tuesday said air defence had shot down missiles over the central province of Homs, with the strikes reportedly targeting regime air bases.

It was not known who carried out the attack, with Pentagon spokeswoman Heather Babb saying: “There are no US or coalition operations in that area.”

The weekend missile strikes by the US, Britain and France were in response to an alleged chlorine and sarin gas attack in Douma on April 7 in which 40 people were said to have been killed.

OPCW director general Ahmet Uzumcu said his nine-strong, all-volunteer team had reached Damascus, but so far “the team has not yet deployed to Douma”.

Syrian and Russian officials had warned of “pending security issues to be worked out before any deployment could take place”, Uzumcu said.

Igor Kirillov, head of Russia’s radiological, biological and chemical protection unit, told reporters the roads still had to be de-mined and cleared and would be tested by UN security services on Tuesday.

“On Wednesday is when we plan the arrival of the OPCW experts,” he added at a press conference in the Russian embassy in The Hague.

The US ambassador to the OPCW, Ken Ward, claimed however the Russians had already visited the site and “may have tampered with it with the intent of thwarting the efforts of the OPCW fact-finding mission”.

The Kremlin dismissed the claims.