Russia says Chemical weapons inspectors in Syria are to be given access to an alleged chemical attack site on Wednesday.

The team has been in the country since Saturday but has been denied access to the site in the town of Douma.

US officials have raised concerns that Russia may have tampered with the site while inspectors were denied access.

Syria and its ally Russia deny responsibility for the 7 April attack. Russia claims that it was “staged”.

Activists on the ground in Syria say the attack killed more than 40 people and injured hundreds more sheltering from bombing in basements beneath the city. Video footage and witness testimony suggests that gas seeped down into the basements, suffocating the victims.

In response to the attack, the US, UK and France carried out targeted military strikes against Syrian chemical weapons facilities.

Reports say Russia and Syria had cited “pending security issues to be worked out” while inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) were denied access to the site of the alleged attack.

Syrian authorities instead offered the inspectors 22 witnesses who they said were at the location of the strike and could be brought to Damascus for interviews.

By the time inspectors arrive at the site on Wednesday, it will have been 11 days since the attack. They are expected to gather soil and other samples to help identify any chemicals used in the suspected attack.

US Ambassador Kenneth Ward said at an OPCW meeting at The Hague on Monday that there were fears Russian forces had tampered with the site during the delay.

“It is our understanding the Russians may have visited the attack site,” Mr Ward said. “It is our concern that they may have tampered with it with the intent of thwarting the efforts of the OPCW Fact-Finding Mission to conduct an effective investigation,” said Ambassador Ward.

Russian Foreign Minister Sergei Lavrov denied any interference with the evidence. “I can guarantee that Russia has not tampered with the site,” he told the BBC.

He said evidence of the chemical attack was “based on media reports and social media” and was a “staged thing”.