* Opposition seeks release of around 50,000 detainees
* Government says talks should focus on broader crisis
* Mediator to meet sides separately - opposition
* Situation is extremely grave - Russia
* UN agency complains about problems in delivering aid
Face-to-face talks between Syria's warring parties stalled on Sunday over easing the humanitarian crisis, opposition delegates said, deepening doubts over tougher political negotiations which are due to follow.
Government and opposition delegations discussed aid and prisoner releases during a morning session in Geneva which had aimed to build some kind of trust between the sides who are implacably at odds over the fate of President Bashar al-Assad.
However, they disputed even the basic facts, and the opposition delegate told Reuters that international mediator Lakhdar Brahimi met the two parties separately later in the day. These sessions would prepare for the more contentious political talks on Monday on the 2012 "Geneva 1" accord, the delegate, Ahmad Ramadan, told Reuters.
Geneva 1 called for the establishment of a transitional governing body in Syria by mutual consent. The opposition says that means Assad must go, a demand the government has dismissed out of hand, adding to pessimism over whether the Geneva talks can make much progress on ending the civil war.
Following the first face-to-face talks on Saturday and Sunday morning, Ramadan said the government side had yet to respond to opposition demands to release thousands of prisoners taken during almost three years of conflict and to allow humanitarian aid into the city of Homs.
"It has been decided that there will be two preparatory separate sessions in the afternoon for Geneva 1 talks tomorrow," he said. "These sessions will be about political negotiations and about the agenda of implementing Geneva 1 with all its articles so that we can start talks tomorrow."
Another opposition delegate confirmed Ramadan's comments.
Russia, one of the talks' sponsors, had said any agreement on easing the humanitarian crisis would help to improve the atmosphere in Geneva, but acknowledged that positions were polarised, emotions were on edge and the situation remained extremely grave.
Underlining the immense difficulty of implementing even local agreements on the ground, a U.N. agency trying to deliver aid to a besieged district of Damascus said state checkpoint officials had hampered its work, despite government assurances it would allow the distributions.
In Geneva opposition figures said they presented a list of 47,000 detainees whose release they are seeking, as well as 2,500 women and children whose freedom they say is a priority.
They also wanted the government to allow aid into the rebel-held centre of Homs, besieged by Assad's forces for 18 months, where the opposition says 500 families urgently need food and medicine.
However, Damascus denied having even got the list. Syrian TV cited a government source as saying Damascus was ready to release any civilians "but the coalition of what is known as the opposition has refrained from presenting a list".
Homs was one of the early centres of protest against Assad's rule which erupted in 2011 before Syria slid into civil war. Since the start of the crisis more than 130,000 people have been killed, two million have become refugees and around half the country needs aid, the United Nations says.
Russian Foreign Minister Sergei Lavrov said earlier he hoped the talks could be conducted in a more business-like manner after the two sides exchanged bitter recriminations during a preliminary day of speeches last Wednesday.
In an interview with NTV television, he called for progress on aid, unblocking besieged areas and prisoner exchanges.
"All this would strengthen trust and affect the atmosphere at the talks in Geneva. Beyond this it is very difficult to make guesses; the situation is extremely grave, positions are polarised, emotions are on the edge," he said in comments posted on the Foreign Ministry's web site.
Humanitarian efforts in Syria have been hindered by fighting and by combatants on both sides, who often try to block deliveries to areas held by their opponents.
The U.N. Relief and Works Agency (UNRWA) complained about problems in delivering deliver food and other aid to Yarmouk, a district of Damascus that is home to impoverished Syrians and Palestinians despite government assurances.
"The agency is extremely disappointed that - at this point - the assurances given by authorities have not been backed by action on the ground to facilitate regular, rapid entry into Yarmouk," spokesman Chris Gunness said.
Faced with finding common ground between two intractably opposed parties, Brahimi had dedicated the first two days of talks to humanitarian issues, hoping to create a platform on which to build the far tougher political talks. He plans to start addressing on Monday what he has called the core issue of the talks - implementing the June 2012 accord.
GENEVA ACCORD "NOT SACRED"
Brahimi described Saturday's sessions as a good beginning but conceded that little progress was achieved, and government delegates took issue with his strategy of addressing specific issues instead of the overall conflict.
"The other side came here to discuss a small problem here or there. We came to discuss the future of Syria," presidential adviser Bouthaina Shaaban said outside the United Nations headquarters in Geneva, where the two sides have held indirect talks - gathered in the same room across two sides of a U-shaped table, but addressing their remarks through Brahimi.
"We did not come here to bring relief to a region here or a region there. We came here to restore safety and security to our country," she told reporters.
Shaaban said the government would not veto any topic and was ready to discuss the 2012 Geneva accord, but that "doesn't mean every word of Geneva is sacred".
The statement was issued 18 months ago under very different circumstances, Shaaban said, also accusing the opposition of focusing exclusively on demands for a transfer of power and ignoring its call for an end to violence.
"Geneva is not the Koran, it's not the Gospel," she told reporters. "Geneva was issued in June 2012. We are now January 26, 2014. The ground has changed. We change according to what this reality requires."
In the summer of 2012 rebel forces took control of much of Aleppo, Syria's biggest city, and were challenging Assad's forces on the edge of Damascus. Since then, backed by Lebanese Hezbollah fighters and Iranian military commanders, Assad's forces have halted the rebel advances and consolidated control over the centre of the country, although there is little sign of them retaking swaths of rebel-held territory in the east.
Syrian Information Minister Omran Zoabi said there was no chance of Assad surrendering power. "If anybody thinks or believes that there is a possibility for what is called the stepping down of President Bashar al-Assad, they live in a mythical world and let them stay in Alice in Wonderland."
Profound mutual mistrust and the absence from Geneva of powerful Islamist opposition groups make any substantial progress very difficult, and previous aid deals and ceasefires in Syria have proved short-lived.
There are now hundreds of rebel groups across the country, including hardline Islamists and al Qaeda-linked militants. Few pay heed to the opposition in exile and the powerful Islamic Front has said negotiators who return from Geneva without having assured Assad's downfall will be treated as traitors.