U.N. Secretary General Ban Ki-moon announced the inquiry on Thursday and made clear it would focus on a rocket attack that killed 26 people near Aleppo. Syria's government and opponents accused each other of firing a missile laden with chemicals.
Russian Deputy Foreign Minister Gennady Gatilov said the investigation could only be objective if it is conducted by a "balanced group of international experts".
The group "must without fail include representatives of the five permanent U.N. Security Council members, including Russian and Chinese chemical specialists", he said on Twitter.
Ban's announcement followed a dispute between Russia and Western council members over the scope of the investigation.
After France and Britain wrote to Ban to draw his attention to a second alleged attack near Damascus and one in Homs in December, both of which rebels blame on the government, Russia accused them of trying to delay the inquiry.
U.S. and European officials say there is no evidence of a chemical weapons attack. If one is confirmed, it would be the first use of such weapons in the two-year-old Syrian conflict, which the United Nations says has cost 70,000 lives.
Moscow initially accused rebels of using chemical weapons in the Aleppo incident, echoing the Syrian government version, but Gatilov later said there was no "unequivocal evidence" of this.
Russia has criticized Western and Arab calls for Assad to leave power and, together with China, has blocked three U.N. Security Council resolutions meant to pressure him to end violence. It has also differed with the West over which side was to blame for alleged massacres and other atrocities in Syria.
Damascus has not confirmed that it has chemical weapons, but says if it had them it would not use them on its own people.
Israeli Major General Yair Golan, who commands forces along the Syrian and Lebanese fronts has said Syria's chemical arsenal is still under the control of the Damascus government.
"(Syria's) chemical weapons today are 100 percent under control. We can say this is good news for now, if you can call it good news," the head of Israel's northern command told the Israel Hayom newspaper in an interview published on Monday.
Israel, which occupies Syria's Golan Heights, has been on alert for spillover from the conflict next door. It said its forces had fired into Syria on Sunday, destroying a machinegun position that had shot at Israeli soldiers.