Syria's opposing parties discussed aid and prisoner releases on Sunday in talks aimed at building some kind of trust before tough political negotiations, but there was no sign of the early progress sought by an international mediator.

Russia, one of the talks' sponsors, said any agreement on easing the humanitarian crisis created by the Syrian civil war would help to improve the atmosphere at the Geneva talks, but acknowledged that positions were polarized, emotions were on edge and the situation remained extremely grave.

Following their first face-to-face talks on Saturday, government and opposition representatives met again in the presence of mediator Lakhdar Brahimi on Sunday morning and were due to hold a second session later in the day.

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.

Opposition delegate Monzer Akbik said the government had promised to answer a request for aid to be let into the rebel-held centre of Homs city, besieged by President Bashar al-Assad's forces for 18 months, where the opposition says 500 families urgently need food and medicine.

"The regime said they will have to go back to Damascus to make a final decision on this and they will give an answer later today," Akbik said. "This is a stalling technique ... We have noticed lack of seriousness from the regime side."

Homs was one of the early centers of protest against Assad's rule which erupted in 2011 before Syria slid into the civil war that killed more than 130,000 people.

Russian Foreign Minister Sergei Lavrov said 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.