Syrian government and opposition negotiators are to hold separate preparatory political discussions with international mediator Lakhdar Brahimi on Sunday after humanitarian talks stalled, opposition delegate Ahmad Ramadan said.

Ramadan said the government delegation had yet to respond to opposition demands to release thousands of prisoners taken during almost three years of conflict and to allow humanitarian aid into the city of Homs.

"It has been decided that there will be two preparatory separate sessions in the afternoon (to prepare the ground) for Geneva 1 talks tomorrow," he said, referring to a 2012 accord which called for the establishment of a transitional governing body in Syria. "These sessions will be about political negotiations and about the agenda of implementing Geneva 1 with all its articles so that we can start talks tomorrow."