Russia has denied interfering with evidence at the site of the suspected Syrian chemical attack which led to Western air strikes on Saturday.
In an interview for BBC's Hardtalk, Russian Foreign Minister Sergei Lavrov said: "I can guarantee that Russia has not tampered with the site."
Concern about tampering was raised by the US envoy to the international chemical weapons watchdog.
International inspectors are trying to reach the site in Douma, near Damascus.
The UK's envoy to the watchdog, the OPCW (Organisation for the Prohibition of Chemical Weapons), has called for the inspectors to be given "unfettered access" to Douma, a town that was a rebel stronghold at the time of the attack on 7 April and is now under the control of the Syrian government and Russian military.
Speaking as the OPCW gathered in The Hague to discuss the crisis, Peter Wilson dismissed as "ludicrous" Russian suggestions that Britain had helped stage a fake attack.