The office of the United Nations Secretary General had no immediate response to that claim, which Western diplomats refuted.
The O.P.C.W. declined to comment, and did not even say whether its inspectors had reached Douma, saying in a statement, “We are unable to share operational details.”
Western diplomats have expressed concern that Syria and its allies could be trying to remove evidence of the assault before inspectors arrive. In an interview with the BBC, Sergey V. Lavrov, the Russian foreign minister, said, “I can guarantee that Russia has not tampered with the site.”
The United States and its allies Britain and France used missiles on Saturday to strike targets in Syria that they said were involved in producing chemical weapons, which President Bashar al-Assad’s government denies having used. Even before the O.P.C.W. inspectors arrived in Syria, the Western allies said they had ample evidence that the country had dropped a chemical agent on Douma, and that it had used chemical weapons many times during the seven-year civil war.
East-West tensions have been high since Britain accused Russia of using a powerful nerve agent to poison a Russian former spy living in England and his daughter. The Kremlin has denied the accusation, which set off the expulsion of hundreds of diplomats and a series of economic measures against Russia.
“It is the style of today’s London to blame Moscow for everything and ascribe certain actions to which we bear no relation whatsoever,” Mr. Ryabkov said.
Russian and Syrian officials have suggested that the chemical attack in Douma never occurred, or that it was staged by rebel forces or Western powers as an excuse for attacking Syria.