The UN Security Council will vote on Wednesday on a draft resolution demanding that the Syrian government cooperate with an investigation of the suspected chemical attack last week in Idlib province, US diplomats said.

Britain, France and the United States put forward the measure which was slightly revised from a previous text presented last week in response to the suspected sarin gas attack in the rebel-held town of Khan Sheikhoun.

The vote is scheduled for 3pm (1900 GMT) and comes as Rex Tillerson, the US secretary of state, is in Moscow to ratchet up pressure on Russia to reconsider its support for Syrian president Bashar al-Assad.