Syria's warring sides have agreed to meet together on Saturday after a first day of peace talks in Geneva stumbled with President Bashar al-Assad's regime threatening to walk away.

After meeting with delegations from the regime and opposition, UN-Arab League Syria envoy Lakhdar Brahimi (pix) said they had agreed to "meet in the same room" after failing to do so on the first day of planned negotiations.

Pulled together by the United Nations, Russia and the United States, the delegations had been due to sit down early Friday at UN headquarters in Geneva for their first face-to-face talks.

But plans for the meeting fell apart after the opposition insisted the regime must be prepared to discuss Assad leaving power.

"We knew that it was going to be difficult, complicated," Brahimi said. "We never expected this to be easy – I think the two parties understand what is at stake."

Foreign Minister Walid Muallem had earlier warned Brahimi that the Syrian delegation "will leave Geneva" should "serious sessions" fail to take place on Saturday.

Muallem told Brahimi "the Syrian delegation is serious and ready to start, but the other side is not," Syrian state television reported.

Still, Brahimi appeared confident no one would be immediately quitting the talks.

"Both parties are going to be here tomorrow and they will be meeting. Nobody will be leaving on Saturday and nobody will be leaving on Sunday," he said.

Brahimi said discussions so far had been "encouraging" but that talks on concrete issues had not yet begun.

"We have not discussed the core matters yet. We hope that both parties will give concessions that will be to the benefit of the process," he said.

Syrian Deputy Foreign Minister Faisal Muqdad told reporters the opposition was the obstacle to talks.

"The problem is that these people do not want to make peace, they are coming here with pre-conditions," he told reporters.

"Of course we are ready to sit in the same room. Why are we coming here then?"

Nazir al-Hakim, a member of the opposition delegation, told AFP it was only willing to negotiate on the basis of the agreement reached at the "Geneva I" peace conference in 2012, which called for the creation of a transitional government.

"We agree to negotiate on the application of Geneva I. The regime does not accept that," he said.

"We will be in the same room when there is a clear agenda for negotiations. We need guarantees that Geneva I will be discussed," Hakim said.

The regime rejects the opposition's contention that the Geneva I agreement requires Assad to go.

Brahimi admitted there were "some differences on the interpretation" of parts of the document.

"Among the many things we hope to be able to achieve is to clarify the ambiguity on those points," he said.

Low expectations for breakthrough

Expectations are very low for a breakthrough at the Geneva II discussions, which officials have said could last up to 10 days.

But diplomats believe that simply bringing the two sides together for the first time is a mark of progress and could be an important first step.

With no one appearing ready for serious concessions, mediators will be looking for short-term deals to keep the process moving forward, including on localised ceasefires, freer humanitarian access and prisoner exchanges.

The opposition arrived in Switzerland with a sole aim – toppling Assad – while the regime says any talk of removing the Syrian leader is a "red line" it will not cross.

Prominent opposition member Burhan Ghalioun told reporters that Assad's opponents would not leave Geneva before the regime.

"We will stay the whole time. The Syrian opposition will not withdaw from this conference until the rights of the Syrian people are met," he said.

The start of the conference in the Swiss town of Montreux on Wednesday was marked by fiery exchanges, with Muallem labelling the opposition "traitors" and agents of foreign governments.

Erupting after the regime cracked down on protests inspired by the Arab Spring, Syria's civil war has claimed more than 130,000 lives and forced millions from their homes.

Pitting Assad's regime, dominated by the Alawite offshoot of Shiite Islam, against largely Sunni Muslim rebels, the war has unsettled large parts of the Middle East.

It took months of efforts to convince the two sides to come to the conference, with the opposition National Coalition only deciding at the last minute to attend.