By Michelle Nichols
UNITED NATIONS (Reuters) - 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.
The senior diplomat said that last week the Chinese had been prepared to abstain in a vote. Russia is one of five council veto powers, along with China, the United States, Britain and France.
"We cannot give up and we must try in good faith, the best we can, to have a text of the Security Council condemning the attack, asking for a thorough investigation," French U.N. Ambassador Francois Delattre told reporters on Tuesday.
An Organisation for the Prohibition of Chemical Weapons (OPCW) fact-finding mission is already investigating the attack in the town of Khan Sheikhoun in a rebel-held area of northern Syria.
If they determine chemical weapons were used, then a joint U.N./OPCW investigation will look at the incident to determine who is to blame. This team has already found Syrian government forces were responsible for three chlorine gas attacks in 2014 and 2015 and Islamic State militants used mustard gas.
In February, Syrian ally Russia, backed by China, cast its seventh veto to protect Assad's government from council action, blocking a bid by Western powers to impose sanctions over accusations of chemical weapons attacks. China has vetoed six resolutions on Syria.
(Reporting by Michelle Nichols; Editing by Chizu Nomiyama and James Dalgleish)