Syria’s regime and opposition have agreed to meet together in the same room for peace talks in Geneva, UN mediator Lakhdar Brahimi says.

“I met the delegations of the opposition and the government separately yesterday (Thursday) and again today (Friday) and tomorrow (Saturday) we expect, we have agreed, that we will meet in the same room,” Brahimi told journalists.

Pulled together by the UN, Russia and the US, delegations from President Bashar al-Assad’s regime and the opposition had been due to sit down early on Friday at UN headquarters in Geneva for their first face-to-face talks.

But Brahimi was unable to convince them to sit together, 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.”

The regime has threatened to withdraw from the talks should “serious sessions” fail to take place on Saturday, but Brahimi appeared confident no one would be immediately walking away from 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, stressing that the sides had “absolutely” agreed to talk face-to-face.

“We hope that it will be a good beginning and that we will continue until the end of next week,” he said, adding that at some point the parties would likely take a break from the discussions for a few days before continuing.

Brahimi said the talks so far had been “encouraging” but said negotiations 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.

He said both sides had agreed that the negotiations would be based on an agreement reached at the “Geneva I” peace conference in 2012, but acknowledged that there were “some differences on the interpretation of some of its items” of the document.

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

The Geneva I agreement calls for the creation of a “transitional governing body”, but it does not specifically spell out Assad’s future role or lack thereof.

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.

The regime has also stressed the Geneva communique’s call to “end to the violence” in Syria, maintaining that the accord’s main focus is reining in “terrorism”, something it accuses the opposition and its international backers of promoting.