UNITED NATIONS — An international inquiry has found that Syrian government forces were responsible for a third toxic gas attack, according to a confidential report submitted to the United Nations Security Council on Friday. The finding sets the stage for a showdown between Russia and Western members of the Council over how to respond.

The fourth report from the 13-month 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 Qminas in Idlib Province on March 16, 2015.

The third report by the inquiry in August blamed the Syrian government for two chlorine attacks — in Talmenes on April 21, 2014, and in Sarmeen on March 16, 2015 — and it said that Islamic State militants had used a mustard agent.

The resulting dispute among the five veto-wielding powers on the Council will most likely pit Russia and China against the United States, Britain and France over how those responsible should be held accountable.