Former UN chief Kofi Annan will on Wednesday launch a diplomatic 'mission impossible' aiming to convince Syrian leader Bashar al-Assad to silence the guns blamed for thousands of deaths in the past year.

Annan will hold talks with Arab League leaders in Cairo in a last chance to send a signal of his intentions before he heads to Damascus on Saturday as joint special envoy for the United Nations and the 22-nation Arab bloc.

Few hold out any chance that he will be able to halt the conflict, which will be one year old on 15 March.

The International Crisis Group think tank said it is offering "very long odds." The chances for diplomacy in Syria are "slim," added Richard Gowan of New York University's Center on International Cooperation.

Annan himself called it "a tough challenge" last week when he was named.

But the Nobel Peace Prize winner is seen as one of the few international figures with the diplomatic muscle to pull off such a coup.

"Syria looks desperate. But the outside world is now desperate. Even if there is only a five percent chance, we have to try," said one Western envoy at the UN.

Annan had regular contacts with Assad when he was secretary general though he admitted they have not spoken for some time. His criticism of NATO's action in Libya last year could also give him credibility with Syria and its dwindling band of supporters.

The immediate aim of the first Damascus talks, however, will be to secure a humanitarian pause in the fighting and access to the protest cities where the UN says more than 7,500 people have been killed.

Then, Annan said, he would "work with the Syrians in coming up with a peaceful solution which respects their aspirations and eventually stabilizes the country."

Annan has insisted that he must be the only international mediator dealing with Assad, a message backed by current UN leader Ban Ki-moon who highlighted the "immense challenges" and said his predecessor "needs the full and undivided support of the international community."

He will in particular need the backing of Russia and China, which vetoed two UN Security Council resolutions on Syria. Both have their own envoys in the region.

Li Huaxin, China's former ambassador to Syria, will be in Damascus on Wednesday. Russia's Foreign Minister Sergei Lavrov will be in Cairo on Saturday to meet Arab League chief Nabil el-Arabi.

With Arab opinion turning against Assad, pressure is growing on them to condemn the violence that Ban called "atrocious" and "grisly".

The UN, and diplomats, say they do not expect to see Russia or China launch initiatives that go against Annan.

"I suspect there will be cooperation with Mr Annan. Mr Annan will probably take into account what they are doing and he will be discussing the situation with them," said deputy UN spokesman Eduardo del Buey.

Annan also has to cope with the tough line from some Arab nations on Syria -- particularly Qatar and Saudi Arabia. An Arab League plan adopted on 22 January calls for Assad to hand over power to a deputy. Russia, in particular, vehemently opposes any such idea.

Ban indicated however that his super-envoy would not just press this plan.

Annan would have "broader flexibility" to have a better chance of getting through to the Damascus government, Ban said. "This is what we have agreed, rather than sticking to any specific point" in the Arab League plan, he said.

So Annan is considered to have the diplomatic muscle, but Assad seems just as determined not to end his assault or step down.

"Annan faces very long odds," said International Crisis Group president Louise Arbour. "The regime seems determined to crush the protest movement and views any concession as a first step toward its downfall."

The special envoy "is taking a major risk in trying to deal with Assad. For Ban, this crisis may define his second term as secretary general. Both surely know that the chances for diplomacy in Syria are slim," said Gowan at New York University.