In an interview for BBC's Hardtalk, Russian Foreign Minister Sergei Lavrov said: "I can guarantee that Russia has not tampered with the site."
The US raised concern at the international chemical weapons agency.
International inspectors will be allowed to visit the site on Wednesday, the Russian military has announced.
A nine-strong team from the agency, the OPCW (Organisation for the Prohibition of Chemical Weapons), has been waiting nearby in the Syrian capital Damascus for the green light.
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.
UK Prime Minister Theresa May has defended Britain's involvement in the air strikes, saying it was to prevent "further human suffering", as opposition parties said MPs should have been consulted in advance.
Catch up on the aftermath of the air strikes:
What happened in Douma?
The suspected attack, denied by Syrian President Bashar al-Assad's government, reportedly killed dozens of people in Douma, in the Eastern Ghouta region.
Two bombs filled with chemicals were reportedly dropped several hours apart on the town.