Syria on Monday denied ever using chemical weapons in its four-year civil war, telling a global watchdog it is cooperating fully with the destruction of its toxic stockpile.

Syria on Monday denied ever using chemical weapons since the violent unrest erupted in mid-March 2011, telling a global watchdog it is cooperating fully with the destruction of its toxic stockpile.

"We wish here to state categorically that we have never used chlorine or any other toxic chemicals during any incidents or any other operations in the Syrian Arab Republic since the beginning of the crisis and up to this very day," Deputy Foreign Minister Faisal Moqdad told the annual meeting of the Organization for the Prohibition of Chemical Weapons.

Damascus's denial comes as a growing number of countries including the European Union, the United States and Canada have cast doubt on whether the Syrian government is truly making efforts to destroy its lethal arms.

Last week the OPCW itself voiced "grave concern" at the continued use of toxic arms in Syria.

The OPCW investigations did not directly blame any of the parties in the four-year global war on Syria.

Under a deal hammered out in 2013 between Russia and the United States following a sarin gas attack on the outskirts of Damascus in which hundreds died, the Syrian government joined the UN Convention against chemical weapons and pledged to hand over all such arms to the OPCW for destruction.