The UN Special Envoy for Syria has said peace talks will continue next week despite the main opposition's decision to leave early.

Syrian chief negotiator Asaad Zoubi said all members of the main Syrian opposition will leave peace talks in Geneva by Friday.

He said there is little prospect of talks resuming unless the situation radically changes on the ground. But Staffan de Mistura dismissed the move as "diplomatic posturing".

The latest round of talks, which began on April 13, was meant to focus on the thorny issue of political transition. Meanwhile, the United Nations said it would name a coordinator to probe the fate of Syrian detainees.

More than 100,000 people are believed to be languishing in government detention centers after five years of civil war. An unknown number are held by rebel and jihadi groups.