The Syrian opposition cast doubt on whether it would go to peace talks planned for Friday, throwing UN diplomatic efforts into question and accusing the United States of adopting Iranian and Russian ideas for solving the conflict.

The Saudi-backed opposition was meeting on Tuesday to decide whether to attend the talks which UN envoy Staffan de Mistura aims to open in Geneva on Friday, ushering in months of negotiations with delegates in separate rooms.

Opposition official Asaad al-Zoubi told Arabic news channel Al-Hadath that he was pessimistic, though the final decision would be taken at the opposition meeting in Riyadh.

De Mistura issued invitations to the talks on Tuesday, although the UN won't confirm the names of the invitees until they confirm their participation.

"It's going to be very low-key proximity talks," UN spokesman Ahmad Fawzi told journalists in Geneva.

A Western diplomat said the aim was to get the talks started without further delay. "There is a little bit of fear that if the talks don't start soon they'll never really get going."

The Syrian government, which is taking territory from the rebels with the help of Russian air strikes and Iranian ground forces, has already said it will attend.

Demanding end to bombardments, blockades

The opposition comprising the recently formed High Negotiations Committee (HNC) has however repeatedly said the government and its allies must halt bombardments and lift blockades of besieged areas before they will go to any talks.

Zoubi, who is due to head the opposition delegation to any negotiations, told Reuters that without the implementation of goodwill steps including release of detainees "there will be no negotiations". "This is what the HNC has laid down," he said.

Reflecting opposition misgivings about the process, he told Al-Hadath that U.S. Secretary of State John Kerry had tabled Iranian and Russian ideas about Syria at a recent meeting with opposition leader Riad Hijab.

"It was not comfortable for us for America - even in theory or partially - to adopt what came in the Iranian and Russian initiatives," Zoubi said in the interview.

He also heaped criticism on de Mistura, saying the UN Syria envoy "cannot impose conditions" on the opposition. The U.S. Special Envoy for Syria, Michael Ratney, urged the opposition to attend the talks.

"Our advice to the Syrian opposition is to take advantage of this opportunity to put the intentions of the regime to the test and to expose in front of international public opinion which are the parties serious in reaching a political settlement in Syria and which are not," he said.