A diplomatic source told Reuters in Astana that the rebel delegation had not yet arrived at the venue. The fourth round of the negotiations started in Astana on Wednesday.
The Syrian armed opposition said Wednesday it had suspended its participation in the two-day talks and demanded an end to government bombing of areas under its control.
Diplomatic efforts to end fighting in Syria have gained momentum in recent months with the announcement of a ceasefire in the Arab country in early January.
The truce excludes terrorist groups such as Daesh (ISIL or ISIS) and Jabhat Fatah al-Sham.
Syria has been gripped by civil war since March 2011 with various terrorist groups controlling parts of it.
According to a report by the Syrian Center for Policy Research, the conflict has claimed the lives of over 470,000 people, injured 1.9 million others, and displaced nearly half of the country’s pre-war population of about 23 million within or beyond its borders.