Syrian government and opposition delegations are discussing prisoner releases on their second day of face-to-face peace talks in Geneva.
On Saturday the talks focused on safe routes for aid convoys into besieged parts of the city of Homs.
UN mediator Lakhdar Brahimi called the discussions a "good beginning".
He said the issues being discussed were a prelude to tackling the key question of establishing a transitional government for Syria.
But the talks have yet to produce any of the confidence-building agreements that that it was hoped might pave the way for discussion later in the week of such wider issues.
The opposition says the government side is stalling over the issue of aid to Homs.
But Syrian presidential adviser Bouthaina Shaaban told the BBC on Sunday it was not the Syrian government preventing aid and medical assistance reaching people, "it is terrorists who are holding civilians as human shields".
Of the prisoner issue, she said: "We have thousands of people who have been kidnapped in Syria and whose families do not know their fate. We want to see who do they have and whether they can set any of those people free."
She added: "I am optimistic in the sense that I know that our delegation... is going to try its utmost to stop this terrorism in Syria, to try and restore peace and security to Syria and then to launch a political process that the Syrian people want.
"But these people really don't represent the opposition. We can't discuss the future of Syria with a small group of people who we doubt, who do they represent."
Monzer Akbi, chief of staff to the president of the opposition National Coalition, accused the government of systematically torturing tens of thousands of people.
"Today, as we are seeking the end of the nightmare of our country, there are thousands of families are wondering where their loved ones are, their children, their sons, wives, fathers, brothers, sisters.
"They have been kidnapped by the regime police forces and they don't know where they are.
"They are crying every day wondering if they are being tortured and they look like that pictures that we saw a few days ago. This is what is being discussed now and negotiated on the table."
Mr Brahimi said he was trying to build confidence by broaching humanitarian issues first.
"We are moving in half-steps," he said, adding that by trying to make progress on humanitarian issues, he was aiming to "create an atmosphere" that would eventually enable advances on the 2012 Geneva I communique, which set out a blueprint for peace talks.
In the first meeting, the two delegations filed in through separate doors into one room in the UN Geneva Headquarters, and sat down at the same U-shaped table, but said nothing to each other.
Mr Brahimi said another two-hour meeting had taken place in the afternoon.
He admitted no direct words had been exchanged but said the two sides were "talking through me to one another.
Syria's civil conflict has claimed well over 100,000 lives since it began in 2011.
The violence has also driven 9.5 million people from their homes, creating a major humanitarian crisis within Syria and for its neighbours.
The opposition and government are fundamentally divided over the aims of the conference.
The government delegation has said the main issue of the talks is finding a solution to foreign-backed "terrorism", by which it means the whole of the armed opposition.
The opposition, however, had insisted that the regime commit in writing to the Geneva I communique, which called for a transition process.
The communique urged Syria to form a transitional governing authority that "could include members of the present government and the opposition and other groups".