A senior Russian diplomat says Russia would welcome the extension of the U.N.-sponsored investigations into chemical weapons use in Syria, but considers it necessary to amend the ground rules for them.

Mikhail Ulyanov, head of the Russian Foreign Ministry's arms control department, harshly criticized a report presented last month by the Joint Investigative Mechanism, or JIM.

The report said experts are "confident" that the Syrian government was behind the sarin attack in the Syrian town of Khan Sheikhoun in April that drew international outrage.

Ulyanov argued Thursday the report was unsubstantiated and ignored evidence suggesting that sarin could have been used by the rebels. He said Russia, which last month vetoed a proposal to extend the JIM's work, will submit its own motion for extending it and making it more objective.