GENEVA: Syria's opposition must accept that it has not won the country's civil war, the UN envoy said Tuesday, voicing hope that the realities of the conflict could help forge a peace deal.

United Nations mediator Staffan de Mistura also stressed that the Syrian government cannot "simply announce victory" as such a call would be hollow without a negotiated settlement to end the six year conflict.

"The only answer is a political process," de Mistura, who has led sputtering UN-backed peace talks, told reporters in Geneva.

In the run up to the next round of negotiations, tentatively scheduled to take place in the Swiss city next month, de Mistura has pushed the still-fractured opposition to form a single delegation that can meet face-to-face with the Damascus regime.

"Will the opposition be able to be unified and realistic enough to realise they didn't win the war?" de Mistura said.

The main opposition camp, the Saudi-backed High Negotiations Committee (HNC), has insisted that Syrian President Bashar al-Assad must leave office as part of any peace deal.

That hardline stance has put the HNC at odds with two more moderate opposition groups, known as the Moscow and Cairo platforms.

De Mistura has in recent weeks upped the pressure on the groups to unite and said he believed they would be meeting again in Riyadh in the coming weeks, after talks there last month failed to produce a compromise.

With Daesh (ISIS) jihadists facing possibly imminent defeat in the key Syrian locations of Deir Ezzor and Raqa, de Mistura raised the prospect of hosting negotiations with Syria relatively free of active fighting.

That would also require an urgently needed "national ceasefire" among the government and various rebels groups, he said.

The UN has helped broker multiple previous truces during the war that has claimed more than 330,000 lives since 2011, but none have proved lasting.