Amman/Beirut: The United Nations invited Syria's government and opposition to peace talks in Geneva on Friday.
The UN Special Envoy for Syria, Staffan de Mistura, sent out invitations to the delayed talks on Tuesday, without saying who had been invited or how many groups might participate.
Earlier, the opposition cast doubt on whether it would go to Geneva, accusing the United States of adopting unacceptable ideas for solving the conflict.
Opposition official Asaad Al Zoubi told Arabic news channel Al Hadath that he was pessimistic, though the final decision would be taken at an opposition meeting in Riyadh.
The Geneva talks are expected to last for months, with diplomats shuttling between rival delegations in separate rooms.
The Syrian government, which is clawing back territory from the rebels with the help of Russian air strikes and foreign ground forces, has already said it will attend.
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 it will join talks.
Zoubi, who is due to head the opposition delegation, told Reuters that without the implementation of goodwill steps including the 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 US Secretary of State John Kerry had tabled foregn countries 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 initiatives," Zoubi said.
He also heaped criticism on de Mistura, saying the UN Syria envoy "cannot impose conditions" on the opposition.
The US 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.
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 United States has supported the opposition to Assad, who it says has lost legitimacy and must leave power. But the opposition has been increasingly critical of US policy. Hijab said earlier this month the United States had backtracked on its position over Syria, softening its stance to accommodate Russia.
Diplomacy has repeatedly failed to resolve the conflict that has killed 250,000 people and forced millions from their homes, spawning a refugee crisis in neighbouring states and Europe. De Mistura is the third international envoy for Syria. His two predecessors - Kofi Annan and Lakhdar Brahimi - both quit.
Preparations for the talks have been beset by problems including a dispute over who should represent the opposition.
Russia has sought to expand the opposition delegation to include a powerful Kurdish faction that controls wide areas of northern Syria. The opposition say the Kurdish PYD party should be part of the government delegation.
PYD leader Saleh Muslim told Reuters he expected his party to be invited to Geneva, though it was unclear in what capacity.
De Mistura has said the Geneva meeting will aim to kick off six months of talks, first seeking a ceasefire, later working towards a political settlement.
Russian Foreign Minister Sergei Lavrov said on Tuesday it would be impossible to reach a peace agreement in Syria without inviting Kurds to take part in the negotiating process.
The Syrian Kurds say the autonomous government they have established in the northeast is a decentralized model for how to resolve the war that has splintered the country.
Turkey, a major sponsor of the insurgency, however said it was against the participation of the Kurdish YPG militia which is affiliated to the PYD. The YPG has become an important partner in the US-led fight against IS.
Turkey views the YPG as an extension of the PKK, which it sees as a terrorist organisation.
The Syrian government and its allies have made significant gains against rebels in western Syria in recent weeks.
On Monday they captured the rebel-held town of Sheikh Maskin in southern Syria near the border with Jordan. It was the first significant gain for Damascus in that area since the start of the Russian intervention on September 30.
In recent weeks government forces and their allies have also captured two strategic towns in the northwestern province of Latakia, where they are trying to seal the border to cut insurgent supply lines to Turkey.