The US on Monday demanded that Syria allow aid into the "starving" city of Homs, as talks aimed at ending three years of civil war hit more trouble over the future of President Bashar Al-Assad.

The Syrian government said women and children could leave the besieged city and that rebels should hand over the names of the men who would remain, but a US State Department spokesman said an evacuation was not an alternative to immediate aid.

"We firmly believe that the Syrian regime must approve the convoys to deliver badly needed humanitarian assistance into the Old City of Homs now," said spokesman Edgar Vasquez. "The situation is desperate and the people are starving."

He said the people of Homs must not be forced to leave their homes and split up their families before receiving food and other aid.

The UN mediator said he hoped Monday's talks in Geneva could tackle the central issue that divides the two sides - Syria's political future and that of Assad - but both sides immediately adopted entrenched positions.

Syria's government delegation presented a document for negotiation which did not mention a transition of power, Syrian television said.

The government's "declaration of basic principles" said Syrians would choose a political system without "imposed formulas" from abroad, an apparent reference to Western and regional demands that Assad step down.

The opposition, which wants Assad to quit as part of arrangements for a transitional government, immediately rejected the proposal.

"The declaration is outside the framework of Geneva, which centres on creating a transitional governing body. It fails to address the core issue," the opposition's chief negotiator, Hadi al Bahra, told Reuters.

Syrian Deputy Foreign Minister Faisal Mekdad said he would meet UN mediators following the opposition's rejection of the government's proposal. "We are here to discuss terrorism, not a transfer of power," he said, using the government's blanket term for the revolt that grew out of peaceful protests in 2011. - Reuters