The latest draft of a resolution being submitted to the U.N. Security Council, and being discussed by ambassadors in New York late on Thursday, has dropped an explicit demand that Assad bows out but still fully supports the "political transition" sought by the Arab League. The change is triggering concerns that the resolution could be drastically watered down to secure agreement.

Diplomatic sources said the main problem was Russian concern that the league plan constituted regime change by another name. "Moscow is looking to fudge this issue of political transition," said one western official.

Language could be further softened during the negotiations, with Britain and other western countries signalling satisfaction with a Russian abstention.

The draft U.N. resolution now looks unlikely to be put to a vote on Friday, pending a crucial meeting of Russia's national security council in Moscow. China is also being tougher than expected.