The U.N. Security Council is set to vote on an extension of the mandate for inspectors charged with determining who is behind chemical weapons attacks in Syria.

The resolution introduced Thursday would extend a joint investigation by the U.N. and Organization for the Prohibition of Chemical Weapons, set to expire on Friday.

Investigators have determined that the Syrian government was behind at least three attacks involving chlorine gas and the Islamic State group was responsible for at least one involving mustard gas.

Western powers 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. Syria has denied its forces used chemical weapons.