UN chemical weapons investigators are expected to return to Syria on Wednesday to continue investigating allegations of chemical weapons use there, Russian Deputy Foreign Minister Sergei Ryabkov said on Tuesday.

"We are pleased that our persistent calls for the return of UN experts to Syria for the investigation of other episodes have borne fruit," Ryabkov said in parliament, referring to alleged incidents of chemical weapons use other than an 21 August attack.

"According to the latest information, the group ... is leaving for Damascus tomorrow, 25 September," he said.