Syria's opposition has postponed its "formal participation" in peace talks in protest over escalating violence, but will remain in Geneva and may continue informal discussions with mediators, the UN envoy said Monday.

"I heard today from the HNC (opposition High Negotiations Committee) their intention to postpone their formal participation in the (UN offices) to the negotiations," United Nations mediator Staffan de Mistura told journalists.

He added that the HNC made the move "to express their own displeasure due to the deterioration of the humanitarian situation" and to protest against surging violence, especially around Syria's second city Aleppo, which has threatened a fragile ceasefire.

But de Mistura insisted that discussions aimed at setting up a new government in Syria would continue through the week.

The HNC does not plan to leave Geneva and may continue to meet UN staff at their hotel "to pursue technical discussions", including on political transition in Syria, de Mistura said.

"We plan, anyway, to continue therefore our discussions and consultations with every side in the (UN's European headquarters) or anywhere else," he said.

De Mistura stressed that the indirect talks format -- which has seen the HNC and President Bashar al-Assad's representatives meet separately with UN mediators -- created flexibility to continue the discussions.

But the opposition's decision to no longer attend official meetings at the UN is the latest blow to the troubled negotiations, which aim to set up a transitional government in Syria within six months ahead fresh presidential and parliamentary elections within 18 months.

So far, de Mistura said, neither side has made compromises on key issues, including Assad's fate.

"I do believe that both sides have been at the moment not yielding a comma, but that is normal in a negotiation," de Mistura said.

"The good news is that both of them are talking about political transition."

The HNC insists that any peace deal must include Assad's departure from power. Damascus has so far said that the president's future is off limits.

"The gap is clearly wide," the UN envoy said after meeting the HNC.