Syria faces "hell" if no deal is struck to end the bloodshed, an international mediator has said.

UN Arab League envoy Lakhdar Brahimi said in Moscow that people inside and outside Syria should "help the Syrians stop their descent into more and more bloodshed, into more and more chaos and perhaps a failed state".

Efforts to find a negotiated solution to a 21-month-old war that has killed some 44,000 people have failed, with the opposition demanding that President Bashar al-Assad be excluded from power before any talks can proceed.