Syria's opposing sides have submitted documents to the United Nations outlining broad principles for a political solution to the country's five-year civil war, the UN envoy said Tuesday.

As peace talks aimed at ending the conflict wrapped up their second day in Geneva, UN mediator Staffan de Mistura told reporters he would try to "analyse" the positions offered by the regime and opposition in a bid to find any possible common ground.

De Mistura made the comment after meeting with the opposition High Negotiations Committee (HNC).

"We... exchanged some papers but also ideas on how to get deeper at the next meeting on the issue of transitional processes," de Mistura said.

The UN envoy has described political transition in Syria as "the mother of all issues" facing the talks.

He gave no details concerning the documents submitted by representatives of Syrian President Bashar al-Assad, whom de Mistura met with on Monday.

But, the regime's lead negotiator, Bashar al-Jaafari, has previously confirmed that Damascus has outlined its general ideas for a political solution to the war, which has killed more than 270,000 people and displaced millions.

In its meeting with de Mistura, the HNC also called for urgent action to address the plight of those allegedly detained by the regime.

HNC spokeswoman Basma Kodmani said the issue of detainees "is not up to negotiation," calling for Damascus to immediately release anyone illegally held.

De Mistura acknowledged that a humanitarian taskforce which has been working to distribute aid since a February 27 ceasefire came into affect had so far made no progress on the issue of detainees.

"On the detainees aspect we have been having extremely, impossibly nothing in terms of outcomes," he said, pledging to push for further action in the coming days.

The talks are set to continue on Wednesday, with de Mistura holding his second meeting with the government side.

He has said regime ally-Russia's partial withdrawal of troops from Syria could be a "positive" development for the talks, with some observers suggesting it could pressure Damascus to negotiate an end to the fighting.