UN mediator Kofi Annan warned on Saturday that the conflict in Syria would enflame the Middle East and beyond unless world powers agreed on a peace plan but the US said foreign ministers meeting in Geneva might fail in that task.

International talks started with governments still in dispute over whether President Bashar al-Assad, reviled in the West for his assault on opponents but still supported by Russia, could have any role in a political transition.

Annan, the former UN chief and the special international envoy on Syria, is hoping for consensus on a plan for a unity government that would exclude controversial figures from leadership - effectively meaning Assad would step down.

"We are here to agree on guidelines and principles for a Syrian-led political transition that meets the legitimate aspirations of the Syrian people," he said in opening remarks.

"No one should be in any doubt as to the extreme dangers posed by the conflict - to Syrians, to the region, and to the world."

His appeal gave a note of urgency to the need for world powers to move closer in their positions as the 16-month-old conflict in Syria deepens by the day.

Moscow, a long-time ally of Assad and an opponent of what it sees as foreign meddling in domestic affairs, objects to any solution imposed on Syria from outside. The US and its European and Arab allies see no way ahead while power remains in Assad's hands.

Annan all but chided the world powers for failing to halt the bloodshed sooner, saying the crisis should never have reached this point.

"Either unite to secure your common interests or divide and surely fail in your own individual way. Without your unity, your common resolve and your action now ... nobody can win and everyone will lose in some way," he said.

The mood of pessimism was reinforced by a senior US official who said the talks might or might not reach a deal on Saturday.

"Discussions remain challenging. We're continuing to work on this today, but we need a plan that is strong and credible. So we may get there, we may not," the official told reporters.

Even as the diplomats gathered at the UN complex by the shores of Lake Geneva, the Syrian army rained mortar fire on pro-opposition areas in Deir al-Zor, Homs, Idlib and the outskirts of Damascus, activists said.

Government troops were fighting rebels of the Free Syria Army at several points. Syria's border with Turkey was also tense following a Turkish military build-up in response to Syria's shooting down of a Turkish warplane last week.

More than 10,000 people have been killed since the anti-Assad uprising broke out and the past few weeks and days have been amongst the bloodiest yet.

Arriving for the talks in Geneva, British Foreign Secretary William Hague said: "It has always been our view that a stable future for Syria, a stable political process means Assad leaving power as part of an agreement on transitional process."

Russian Foreign Minister Sergei Lavrov and US Secretary of State Hillary Clinton met in St. Petersburg, Russia, on Friday night but failed to resolve differences.

"Our Western partners want themselves to decide the outcome of the political process in Syria although it is the job for the Syrians," Lavrov's deputy, Gennady Gatilov, said prior to the Geneva meeting.

The foreign ministers of the five permanent members of the U.N. Security Council - Russia, the US, China, France and Britain - were attending the talks.

Turkey, Kuwait, Qatar, UN Secretary-General Ban Ki-moon and Arab League chief Nabil Elaraby and European Union foreign policy chief Catherine Ashton are also taking part.

Also present was Norwegian Major General Robert Mood, who headed a failed UN ceasefire monitoring mission to Syria and was witness to the violence and suffering on the ground.

However, Iran, Syria's closest regional ally, and Saudi, a foe of both Damascus and Tehran and leading backer of the rebel forces opposing Assad, are not represented. Nor is anyone from the Syrian government or opposition.

According to a draft document from Annan, seen by Reuters, the envoy envisages the setting up a transitional government of national unity which can establish a neutral environment for political change. It would have full executive powers.

"It could include members of the present government and the opposition and other groups, but would exclude from government those whose continued presence and participation would undermine the credibility of the transition and jeopardize stability and reconciliation," it said.

That proposal is the stumbling block as it effectively means Assad cannot be involved, which Moscow views as imposing a solution. The Syrian opposition also demands that he is barred from any role.

The UN's Ban, opening the closed-door meeting, underlined the need to reach an agreement "today" and his appeal was echoed by the Arab League's Elaraby.

The Syrian conflict has evolved from peaceful protests against the Assad family's four-decade rule to something akin to a civil war with a sectarian dimension.

Although the world has condemned the ferocity of Assad's forces' crackdown on the opposition - which includes assaults on pro-opposition areas and mass arrests - it has been unable to halt violence which threatens to draw in more of the region's religious and political rivalries and alliances.

Russia and China have objected to what they see as the Western interference that brought about the downfall of rulers such as Libya's Muammar Gaddafi.

But Western governments have shown little will to repeat last year's Libyan experience of military support for rebels in Syria, where Assad's forces are formidable and the regional complexities greater.

Video posted on Saturday by activists in the eastern desert city of Deir al-Zor showed smoke rising from apartment blocks as explosions rang out, some shaking the camera. Activists also reported shelling in Homs, Idlib and the outskirts of Damascus.

State media agency Sana also reported an assault of the town of Douma, 15 km from Damascus, where activists say more than 50 people have been killed since Thursday. Residents said government forces pushed their way into Douma on Saturday.

"The authorities continued cracking down on armed terrorist groups and raiding the hideouts of terrorists in Douma, killing scores of terrorists and injuring and arresting big numbers," Sana said.

Although the government routinely refers to its enemies as "armed terrorist groups", Assad himself conceded this week that the country was now in a state of war.-Reuters