Syrian rebels and government officials are meeting separately with the UN envoy as a new round of peace talks gets under way in Geneva.
The meetings on May 16 are the sixth round of the talks brokered by UN special envoy Staffan de Mistura.
Five previous rounds of UN-backed negotiations aimed at ending the six-year conflict that has killed more than 320,000 people have failed to yield concrete results.
The Geneva talks have been overshadowed by U.S. State Department accusations on May 15 that the Syrian government built a crematorium in one of its most notorious prisons to cover up dozens of daily executions there.
UN envoy Staffan de Mistura met with Syrian government negotiator Bashar al-Jaafari on May 16, followed by the opposition High Negotiations Committee (HNC).
Jaafari also met with Russian Deputy Foreign Minister Gennady Gatilov, Syria's state news agency SANA reported, before returning to the UN in the evening for further talks. The HNC was also due back later on.
On May 15, the State Department said it believed about 50 detainees are being hanged each day at the Saydnaya military prison, which is located about a 45 minutes' drive north of the capital, Damascus.
Syria's government said it "categorically" denies the U.S. accusations of mass killings at a prison, including executions of political opponents, and burning the victims in a crematorium to hide the evidence.
The Foreign Ministry in Damascus said in a May 16 statement that the U.S. State Department accusations were "a new Hollywood plot" and "lies" that are being used to justify U.S. "aggression and intervention."
The revelations echoed a February report by Amnesty International that said Syria's military police hanged as many as 13,000 people in four years and buried them in mass graves.
However, monitoring groups have not verified the crematorium allegation.
With reporting by Reuters, AP, AFP, and dpa