International peace envoy Kofi Annan warned on Saturday that Syria was slipping into "all-out" war, and a Syrian opposition figure said Russia had become part of the problem and should urge President Bashar al-Assad to quit power.

"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."

Prime Minister Sheikh Hamad bin Jassim al-Thani of Qatar - which has backed arming Sunni Muslim insurgents fighting to topple Assad, a member of Syria's minority Alawite sect - said Annan should set a time limit for his mission.

He also called on the U.N. Security Council to put Annan's plan - based on a ceasefire that has yet to take hold, and that is meant to lead to a negotiated end to the Syrian crisis - under Chapter 7 of the U.N. charter, a measure that could authorise the use of force.

"We want the Security Council to refer the six-point (Annan) plan to a Chapter 7 resolution. We, the international community, cannot accept the situation to continue as it is," he said.