The Syrian government's chemical weapons stockpile has been linked for the first time by laboratory tests to the largest sarin nerve agent attack of the civil war, diplomats and scientists told Reuters, supporting Western claims that government forces under President Bashar al-Assad were behind the atrocity.

Laboratories working for the Organisation for the Prohibition of Chemical Weapons compared samples taken by a UN mission in the Damascus suburb of Ghouta after the August 21, 2013 attack, when hundreds of civilians died of sarin gas poisoning, to chemicals handed over by Damascus for destruction in 2014.

The tests found "markers" in samples taken at Ghouta and at the sites of two other nerve agent attacks, in the towns of Khan Sheikhoun in Idlib governorate on April 4, 2017 and Khan al-Assal, Aleppo, in March 2013, two people involved in the process said.

Meanwhile, Russia's showpiece congress aimed at bringing Syria's seven-year war to an end opened yesterday despite boycotts by the main opposition group and Kurds, as well as rebels pulling out at the last minute, reported AFP.

Regime-backer Moscow has invited 1,600 delegates to the meeting in the Black Sea resort of Sochi as part of a broader push to consolidate its influence in the Middle East and start hammering out a political solution to the conflict.