The UN Security Council has scheduled a vote on a resolution that would condemn the reported use of chemical weapons in northern Syria and demand that all parties provide speedy access for investigators.
Britain's UN Ambassador Matthew Rycroft called on all 15 members to support the resolution drafted by the UK, France and the United States, saying "every country that really wants the truth will vote in favour of the resolution."
But the final draft includes a paragraph that the Russians objected to last week.
It stresses Syria's requirement to provide investigators with flight plans and information about air operations on April 4 when Khan Sheikhoun was attacked - names of helicopter squadron commanders, and immediate access to air bases where they believe an attack may have been launched.
Mr Rycroft said Russia has a choice of sticking with "the toxic Assad regime that poisons its own people" or seeking peace through negotiations and a political transition.
Later, a senior Russian diplomat said Moscow is opposing the latest version of the resolution.
Deputy Foreign Minister Gennady Gatilov told the Interfax news agency that the draft submitted by the US, Britain and France is "unacceptable to us in its current form".
He added that "we will vote against it if our partners do not heed our calls and promote this draft artificially with the sole purpose of making the Russian Federation veto it again".