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 nine-strong team from the watchdog, the OPCW (Organisation for the Prohibition of Chemical Weapons), was told by Syrian and Russian officials in Damascus there were still “security issues to be worked out”, OPCW chief Ahmet Uzumcu said.
Douma 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.
MrUzumcu was speaking as the start of OPCW emergency talks in The Hague.
The UK’s envoy to the OPCW, Peter Wilson, called for the inspectors to be given “unfettered access” to Douma and dismissed as “ludicrous” Russian suggestions that Britain had helped stage a fake attack.