* Government says women, children free to leave Homs
* U.S. demands food aid be allowed into "starving" city
* Opposition seeks release of around 50,000 detainees
* UN mediator hopes political talks can start Monday
By Stephanie Nebehay and Mariam Karouny
GENEVA, Jan 27 (Reuters) - The United States 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. A U.S. 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 aid.
After long months of fighting, much of Syria's third biggest city has been reduced to rubble and people inside are under siege, cut off from supplies.
The city's fate has turned into a test of whether the first peace talks attended by both sides in the three-year war can achieve practical measures on the ground, while a broader political settlement seems as remote as ever.
"Once again, I tell you we never expected any miracle, there are no miracles here. But we will continue and see if progress can be made and when," said U.N. envoy Lakhdar Brahimi, the host of the talks.
Efforts to reach a political settlement have stumbled over the question of creating a transitional government, which the opposition and its Western backers believe would remove Assad from power. Assad's government refuses to discuss it.
Syria's government delegation presented a document for negotiation which did not mention a transition of power.
The government's "declaration of basic principles" said Syrians would choose a political system without "imposed formulas" from abroad. The opposition immediately rejected it.
"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.
Homs, occupying a strategic location in the centre of the country, has been a key battleground, and one of the areas in most urgent need of humanitarian relief. Assad's forces retook many of the surrounding areas last year, leaving rebels under siege in the city centre, along with thousands of civilians.
Residents say they have little hope that the Geneva talks will save them.
"The people know that the talk is only about letting in humanitarian aid," activist Yazan Homsi, contacted in one of the besieged parts of Homs, told Reuters. "They want an end to the siege. To break it.
"Sending aid will only extend the duration of the siege. The aid they are talking about will last for a day or a week or two months. But it won't do more than that," he added. An online video from Homs showed demonstrators with Islamist flags denouncing the Geneva talks as "treachery".
Children play in the rubble that litters the streets. The city's buildings are smashed and its mosques are holed by shell fire. In one deserted souk, debris lies in the aisles and the roof is shredded with bullet holes.
A photo recently posted on Facebook shows a frail boy in Homs holding a poster that reads: "Breaking the siege is a non-negotiable demand."
Syrian Deputy Foreign Minister Faisal Mekdad told a news conference on Sunday the government would let women and children leave the city centre if rebels gave them safe passage.
Western diplomats said the Syrian government should move quickly to allow aid in or face a possible United Nations Security Council resolution, with Russia and China being urged to reverse their opposition to such a move.
"The ball is still in the regime's court. We understand that a report has gone back to Damascus seeking instructions," one diplomat said.
It highlighted one of the difficulties of the Geneva talks - the opposition delegation only represents some of the rebel factions on the ground. Powerful Islamist fighters allied to al Qaeda are not represented at all.
Brahimi said opposition delegates, who have asked for the release of nearly 50,000 detainees, had agreed to a government request to try to provide a list of those held by armed rebel groups - though many of these groups, fighting among themselves, do not recognise the negotiators' authority.
Mekdad said the government had examined an opposition list of 47,000 people believed to have been arrested by Assad's forces and found most had either never been held or were now free. He also denied that any children were being held.
Russian Foreign Minister Sergei Lavrov, whose government has helped Assad resist Western pressure, called for progress on aid, unblocking besieged areas and prisoner exchanges.
Underlining the difficulty of implementing even local agreements on the ground, a U.N. agency trying to deliver aid to a besieged rebel area of Damascus said state checkpoints had hampered its work, despite assurances from the government that it would allow the distributions.
After years when Western countries believed Assad's days were numbered, the past year has seen his government reclaim territory and solidify its position diplomatically.
The United States abruptly called off strikes meant to punish Damascus for suspected use of chemical arms, ending more than two years of speculation that the West might join the war against Assad as it did against Libya's Muammar Gaddafi in 2011.
Under a deal that headed off U.S. strikes, Assad agreed to give up Syria's poison gas. Shipments of toxins for destruction began this month. A U.N. agency said more shipments took place on Monday. (Additional reporting by Stephen Kallin, Erika Solomon and Tom Miles; Writing by Giles Elgood; Editing by Andrew Heavens; Editing by Philippa Fletcher and Peter Graff)