Russia's ambassador to the United Nations says he is unconvinced by a report linking the Syrian government to the use of chemical weapons in at least three instances.

Vitaly Churkin said Thursday that the report lacked sufficient testimony, material proof and was full of contradictions.

Virginia Gamba headed the inspection team under the joint auspices of the U.N. and the Organization for the Prohibition of Chemical Weapons. She defended the investigators' professionalism and said they stand firmly behind their assessment and conclusions.

The United States, Britain and France want the Security Council to impose sanctions on the Syrian government for using chemical weapons. But Russia, Syria's closest ally, has repeatedly questioned investigators' conclusions linking chemical weapons use to the regime of President Bashar Assad.

The report was released Thursday.