The shadow of past international failures in Bosnia and Rwanda hangs over special envoy Kofi Annan and the UN Security Council as they dig in for a prolonged showdown with Syria's President Bashar al-Assad.

Annan, who is expected to brief the Security Council about the Syria crisis again on Tuesday, was head of the UN peacekeeping department from 1993 to 1996 -- the dark years of the Bosnia war and the Rwanda genocide.

The butchering of 8,000 Muslim men and boys in the Bosnian town of Srebrenica in 1995 "will forever haunt the history of the United Nations," he once said. Annan also said he could have "done more" to stop the slaughter of 800,000 people in Rwanda in 1994.

Senior diplomats quote the former UN secretary general as saying he is determined to avoid a repetition of those failures.

"These are episodes which have marked his life," said one UN diplomat who has had contact with Annan over the Syria mission.

"If someone fires on the observers or there are massacres, it will not be like Bosnia -- we will not act as though nothing has happened," the diplomat quoted Annan as saying.

"Annan considers that his main weapon in this showdown is his prestige, his credibility, and he could lose that if there are one or two or three ceasefires which don't work, as there was in Bosnia," added a western envoy.

"There is a certain deja vu quality" to the war in Syria, Annan's spokesman, Ahmad Fawzi, told US media chain McClatchy Newspapers last week. Annan, like the UN-backed negotiators in Bosnia, cannot order force if Assad does not keep his commitments.

The UN-Arab League envoy is earning praise for the way he has kept the international powers behind the six-point peace plan agreed with Assad -- while at the same time keeping pressure on the Syrian leader.

But the UN envoys who in public have spoken so strongly in favor of Annan are now wondering how much longer his mission can go on and what future action can be taken.

The UN Security Council voted on Saturday to send 300 unarmed observers to Syria to monitor a fragile ceasefire. But the United States has already said it may not support the renewal of the mission after the first 90-day mandate.

The United States, Britain and France have called for sanctions if the observers cannot work. That will put them on a collision course with Russia, Syria's last major ally, which fiercely opposes sanctions.

"I'm sure it's going to be a lengthy, painful and difficult process, but there is no alternative. The alternative is a further deterioration of the situation, further bloodshed," said Russia's UN ambassador Vitaly Churkin when appealing for all countries to 'work very doggedly in support of Kofi Annan'.

Given the dim prospects for the observer mission and the likelihood that Russia would veto any UN sanctions, many western capitals are already asking -- what next?

"Annan is a prisoner of his own mediation. He is not likely to say, 'I am stopping because of Syria'," the UN diplomat said.

"There has to be a time when we say, 'Mr Annan, it is not working.' But the question is when? We cannot say that the Annan mission has failed as long as we have not gone to the end of the road. That will be a political decision," the diplomat added.

Tony Blair, the international Middle East conflict mediator and former British prime minister, says that efforts to reach a negotiated solution have to be given a chance.

He told CBS television that he does not expect to see foreign troops in Syria. "But I do expect that if Assad reneges on his commitments and they carry on killing the civilians, then I think we will see a gradual buildup of additional actions like corridors that allow help to get in, possibly secure zones that will protect the Syrian people and Syrian refugees."

A senior UN official, speaking on condition of anonymity, said all these ideas are being considered in 'contingency plans' -- along with the best ways to get UN observers out of Syria in a hurry.