The executive body of the global chemical arms watchdog on Friday took the unprecedented step of condemning Syria and Islamic State jihadists for using toxic weapons, demanding increased inspections.

The leading body of the Organisation for the Prohibition of Chemical Weapons also voiced concern about "gaps and discrepancies" in Syria's 2013 declaration about the size of its toxic arms stockpile.

The resolution, adopted by a rare majority vote, "condemns in the strongest possible terms the use of chemical weapons in the Syrian Arab Republic," the OPCW said in a statement.

It pointed to findings by a recent joint UN and OPCW investigation which concluded that "the Syrian Arab Armed Forces and the so-called Islamic State ... have been involved in the use of chemical weapons".

Sources, who attended the closed door talks, had told AFP it was the first time the watchdog had found a state member to have violated the Chemical Weapons Convention.

But this was not spelled out in the resolution, which nevertheless urged Syria "to comply fully with its obligations," the OPCW statement said.

The joint UN-OPCW panel's report released last month concluded Syrian President Bashar al-Assad's forces had carried out three attacks on villages in 2014 and 2015.

Government helicopters from two regime-controlled air bases dropped chlorine barrel-bombs on the villages of Qmenas, Talmenes and Sarmin, in rebel-held Idlib province.

IS jihadists meanwhile used mustard gas in August 2015 in Syria, the panel determined.

The executive council said those using such weapons must "desist immediately and authorises additional inspections at selected sites... of concern in Syria."

- 'Accountability' -

Based in The Hague, the OPCW usually works by consensus, but after weeks of behind-the-scenes negotiations on the text it became impossible to reach unanimity, mainly due to Russian objections, one source who attended the session told AFP.

When it was clear that "an overwhelming majority" supported the four-page resolution put forward by Spain it was decided to put it to a vote, the source said.

A total of 28 countries including Britain, France and the United States voted in favour, gathering the two-thirds needed to pass, separate sources said.

Four countries voted against -- China, Iran, Russia and Sudan -- while nine countries abstained.

"There is a clear determination across the international community to hold those who have used these heinous weapons to account," said British Foreign Secretary Boris Johnson in a statement.

The OPCW body ordered inspections twice a year at the Barzah and Jamrayah research centres, warning Damascus must "facilitate promptly, and fully cooperate with, these inspections."

Syria officially joined the Chemical Weapons Convention in October 2013, vowing to hand over its stockpile of toxic arms for destruction and undertaking never to use chemical weapons.

After years of denying it possessed chemical weapons, Syria was pushed into the convention under a deal brokered by the United States and Russia, averting a threatened US air strike following an August 2103 gas attack.

All its declared stock of chemical weapons have now been destroyed, but attacks have continued.

Friday's vote again pitted Washington against Moscow. Russia has blocked moves at the UN Security Council to sanction Syria for atrocities committed by Syrian forces on civilians during the five-year war.

The OPCW's resolution will now be submitted to the Security Council, and Syria will likely top the agenda at the annual conference of the watchdog's 192 member states which opens on November 28 in The Hague.