The global chemical weapons watchdog on Wednesday condemned the use of chlorine as a weapon in Syria as a breach of international law and said those responsible should be brought to justice.

The executive council of the Organization for the Prohibition of Chemical Weapons expressed "serious concern" at a fact-finding mission's recent report that concluded "with a high degree of confidence" that chlorine was unleashed on three villages in northern Syria from April to August last year, killing 13 people.

"This decision makes it absolutely clear that chemical weapons are illegal, and that their use will not be tolerated under any circumstances," the organization's director-general, Ahmet Uzumcu, said in a statement.

Chlorine gas is used in industry around the world, but can also be used as a weapon.

The fact-finding mission's report did not apportion blame for the attacks, but said 32 of 37 people interviewed "saw or heard the sound of a helicopter over the village at the time of the attack with barrel bombs containing toxic chemicals."

British Foreign Secretary Philip Hammond said an earlier report's similar findings linking chlorine attacks to helicopters "corroborate allegations that the Assad regime is continuing to use chemical weapons in Syria, in violation of the Chemical Weapons Convention."

Syria denies using chlorine or other chemical weapons and blames "terrorists" for such attacks.

Damascus joined the OPCW in 2013 in a move widely seen as averting U.S. airstrikes in the aftermath of a chemical attack on a Damascus suburb that killed hundreds of civilians.

The U.S. and Western allies accused the Syrian government of being responsible for that attack, while Damascus blamed rebels.

The 1,300-ton chemical weapon arsenal Syria declared when it joined the OPCW has since been destroyed.