The U.N. Security Council could vote as early as Wednesday on a push by the United States, Britain and France to bolster support for international inquiries into a deadly toxic gas attack in Syria, diplomats said, a move Russia had deemed unacceptable and unwarranted.

The three countries have proposed a revised draft resolution, diplomats said, similar to a text they circulated to the 15-member council last week that condemns the April 4 attack and pushes Syria’s government to cooperate with investigators.

Western powers blame the sarin gas attack, which killed scores of civilians – many of them children, on Syrian President Bashar al-Assad’s forces.

Syria’s government has denied responsibility for the attack, which prompted a U.S. strike on a Syrian air base.

The Security Council vote would come at the end of a visit to Moscow by U.S. Secretary of State Rex Tillerson.

A senior Security Council diplomat, speaking on condition of anonymity, said the most likely scenario appeared to be a veto of the U.N. resolution by Syrian ally Russia.

Russia said last week the draft resolution was unacceptable and unwarranted.