Growing doubts about the future of the UN observer mission in Syria will be raised at a UN Security Council meeting on Tuesday as President Bashar al-Assad steps up his brutal crackdown, envoys said.

Major General Robert Mood, head of the UN Supervision Mission in Syria (UNSMIS), is to brief the 15-nation council four days after he halted patrols by unarmed monitors because of the threat to their safety.

A UN spokesman reaffirmed Monday that the monitors had been put at "extreme risk".

Some envoys say it is possible the monitors will have to be brought out before the end of the mission's 90-day mandate, which would be a major blow to efforts by UN-Arab League envoy Kofi Annan to secure a negotiated settlement between President Bashar al-Assad and opposition groups.

"I think what we will want to hear from General Mood is what he thinks the prognosis is for the mission," Britain's UN ambassador Mark Lyall Grant told reporters.

"I think there will be a lot of member states of the council, including us, who will be questioning now what the future is for the mission and, therefore, by extension the Annan plan."

"Obviously we are very concerned about the increasing levels of violence and we lay all the responsibility for that at the door of the Syrian regime," said Lyall Grant.

Asked whether the mandate could be cut short, Lyall Grant said, "I think we are going to have to review it in the light of what has happened, so I don't rule that out entirely, no."

"There are, of course, many questions now being asked about the mission," said a diplomat from another UN Council member, speaking on condition of anonymity.

The United States has already said it was ready to block the renewal of the UNSMIS mandate.

China's UN ambassador Li Baodong said all Security Council members were "strongly concerned" at the suspension of the UN mission.

He told reporters the government and opposition should "offer full cooperation" to UN monitors.

Britain, France and the United States are working on a draft resolution that would propose sanctions against Assad if he does not carry out Annan's six point peace plan.

Under the Annan blueprint, Syrian government forces and heavy weapons were meant to be withdrawn from cities as key steps toward launching political talks. Opposition groups say that a series of massacres blamed on government forces and their militia allies have made talks impossible.

The draft resolution drawn up would propose non-military sanctions under Chapter VII of the UN Charter, according to diplomats.

"Faced with the regime's relentless repression, particularly in the city of Homs which is being shelled by Syrian armed forces, it is more than ever necessary to confer binding force on the Annan plan by putting it under Chapter VII," French foreign ministry spokesman Bernard Valero said in Paris.

Russia, Syria's main international ally, has said it will oppose any approval of military force. Russia and China, both permanent members of the 15-nation council, have twice used their vetoes to block resolutions which just hinted at sanctions.

The United Nations said that Mood, who will speak to the Security Council from 2000 GMT on Tuesday, wanted the monitors to start work again but the level of violence is just too high.

"It's simply untenable that they are either blocked or shot at," a UN spokesman Farhan Haq told reporters, highlighting the "extreme risk" and the "close calls" they have already been involved in.

"The conditions on the ground are such that it is not even simply a question of monitoring whether a cessation of hostilities is holding," said Haq.