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 members of the UNSC over how to respond.

The fourth report from the 13-month-long inquiry by the United Nations and the Organization for the Prohibition of Chemical Weapons, the global chemical weapons watchdog, blamed Syrian government forces for a toxic gas attack in Qmenas in Idlib governorate on March 16, 2015, according to a text of the report seen by Reuters.

The third report from the inquiry, made 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 sulfur mustard gas.