By Anthony Deutsch

AMSTERDAM (Reuters) – The executive body of the global chemical weapons watchdog voted on Friday to condemn the use of banned toxic agents by the Syrian government and by militant group Islamic State, a source who took part in the closed session said.

Roughly two-thirds of the 41 members on the Executive Council of the Organisation for the Prohibition of Chemical Weapons (OPCW), endorsed a U.S.-tabled text, the source told Reuters.

The OPCW’s Executive Council, which meets behind closed doors, seldom votes on such matters, generally operating through consensus. But this text was supported by 28 members, including Germany, France, the United States and Britain.

It was opposed by Russia, China, Sudan and Iran. There were nine abstentions. Russia and Iran are Syrian President Bashar al-Assad’s main allies against rebels seeking to overthrow him. Western and Gulf Arab states back the rebels.

The U.S.-Russian split over Syria was striking as it was those two countries that in 2013 took the lead in getting the Damascus government to join the OPCW and avert threatened U.S.-led military intervention in Syria’s civil war.

A 13-month international inquiry by the OPCW and United Nations concluded in a series of reports that Syrian government forces, including helicopter squadrons, were responsible for the use of chlorine barrel bombs against civilians.

The OPCW-U.N. mission found that the Syrian government carried out three toxic attacks in March and April of last year, while Islamic State militants had used sulfur mustard gas.

The findings set the stage for a U.N. Security Council showdown between the five veto-wielding powers, likely pitting Russia and China against the United States, Britain and France over how those responsible for the attacks should be held accountable.

Syrian authorities deny having used chemical weapons in the conflict. Islamic State has not commented.

(Reporting by Anthony Deutsch; Editing by Mark Heinrich)