An international inquiry found Syrian government forces responsible for a third toxic gas attack, according to a confidential report submitted to the U.N. Security Council on Friday, setting the stage for a showdown between Russia and western council members over how to respond.

The fourth report from the 13-month-long inquiry by the United Nations and the Organisation for the Prohibition of Chemical Weapons (OPCW), the global chemical weapons watchdog, blamed Syrian government forces for a toxic gas attack in Qmenas according to a text of the report seen by Reuters.

Boris Johnson, the foreign secretary, said the report underscored the "cruel barbarity" of Bashar al-Assad's regime. “The use of chemical weapons is horrific, and a breach of international law and UN Security Council resolutions. It is crucial to hold those responsible to account," he said.

The third report by the inquiry in August blamed the Syrian government for two chlorine attacks - in Talmenes on April 21, 2014 and Sarmin on March 16, 2015 - and said Islamic State militants had used sulphur mustard gas.

The results set the stage for a 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 should be held accountable.