"The spectre of an all-out war, with an alarming sectarian dimension, grows by the day," Annan told a meeting of members of the Arab League, co-sponsor with the United Nations of a peace plan aimed at ending the bloodshed in Syria.

Burhan Ghalioun of the Syrian National Council, the divided body claiming to speak for political opposition to Assad, said: "With its support of the regime and for Assad remaining, Russia has become part of the problem rather than part of the solution. If it cooperates to find a formula that makes Assad leave, it will become part of the solution."