THE HAGUE - The executive body of the global chemical arms watchdog took the unprecedented step Friday of condemning Syria and Islamic State militants for using toxic weapons and called for stepped up inspections, sources and officials said.

It is the first time the watchdog has found a state member to have violated the Chemical Weapons Convention, and came during a rare vote by its 41-member executive council, sources who attended the closed session told AFP.

A four-page resolution put forward by Spain voiced "grave concern" over the findings of a one-year investigation by a joint panel of the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW), according to a copy seen by AFP. It "condemns in the strongest possible terms" the use of chemical weapons in Syria and calls on "all parties identified" in the report to "immediately desist from any further use".

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

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

IS militants meanwhile were found to used mustard gas in August 2015 in Syria.

The OPCW resolution pointedly "demands" that Syria "comply fully with its obligations under the convention," and mandated the watchdog as soon as safely possible to carry out inspections at the site of the attacks.

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 resolution 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 of condemning Syria and Islamic State, gathering the two-thirds needed to pass, the sources said.

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

"This decision confirms that the Assad regime and Daesh are responsible for using abhorrent chemical weapons against civilians," said British Foreign Secretary Boris Johnson in a statement after the vote.

"There is a clear determination across the international community to hold those who have used these heinous weapons to account."

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

After years of denying it possessed any chemical weapons, Syria was pushed into the convention under a deal brokered by the United States and Russia, averting a threatened US air strike on its facilities.

All its declared stock of chemical weapons have now been destroyed by the OPCW, but attacks have continued and in July the body's head Ahmet Uzumcu said Damascus had "not yet adequately explained the presence of indicators of four chemical warfare agents".

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.