AMMAN/BEIRUT/GENEVA (Reuters) – The United Nations invited Syria’s government and opposition to peace talks in Geneva on Friday, but Saudi-backed opponents of President Bashar al-Assad have yet to decide whether to drop their objections to taking part.

The U.N. 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 had cast doubt on whether it would go to Geneva, accusing the United States of adopting unacceptable Iranian and Russian ideas on solving the conflict.

A decision by the opposition’s recently formed High Negotiations Committee (HNC) on whether to accept the invitation is due to be taken at a meeting in Riyadh.

“There is consensus in the High Committee on being positive in our decision,” spokesman Salim al-Muslat told the Arabic news channel Arabiya al-Hadath. However, he added that the final decision would not be made at the meeting until Wednesday.

A Western diplomat said the Geneva talks could not be convened if the HNC were to stay away, and de Mistura would have to find a face-saving way to avoid the complete collapse of the process, perhaps by announcing a further delay.

The main Syrian Kurdish party said it had not been invited, a move some interpreted as intended to keep Turkey, which regards Syria’s Kurdish fighters as terrorists, engaged in the process.

De Mistura has said the Geneva meeting will first seek a ceasefire and later work toward a political settlement. The talks are expected to last for six 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 other allies including Iranian fighters, has already said it will attend.

The HNC has however repeatedly said the government and its allies must halt bombardments and lift blockades of besieged areas before it will join talks.

Opposition official Asaad al-Zoubi, who is due to head the HNC delegation, told Reuters that without the implementation of goodwill steps including the release of detainees “there will be no negotiations”.

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 … to adopt what came in the Iranian and Russian initiatives,” Zoubi said.

The U.S. Special Envoy for Syria, Michael Ratney, urged the opposition to attend.

“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.

LOST LEGITIMACY

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 U.S. 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 neighboring 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 tried to expand the opposition delegation to include a powerful Kurdish faction that controls wide areas of northern Syria. The Sunni Arab opposition say the Kurdish PYD party should be part of the government delegation.

Turkey said it would not attend if the PYD was invited.

The PYD, which is fighting Islamic State and has enjoyed military support from the United States, is a terrorist group and has no place with the opposition at the negotiating table, Turkish Foreign Minister Mevlut Cavusoglu said.

PYD leader Saleh Muslim, who earlier told Reuters he expected his party to be asked to Geneva, said he not received an invitation and was not aware that any Kurdish representatives had been asked to come.

Russian Foreign Minister Sergei Lavrov said earlier it would be impossible to reach a peace agreement in Syria without inviting Kurds to join 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.

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 Sept. 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.

(Additional reporting by Lisa Barrington in Beirut, Vladimir Soldatkin and Andrew Osborn in Moscow, Stephanie Nebehay in Geneva and Nick Tattersall in Turkey; Writing by Tom Perry; Editing by Giles Elgood and David Stamp)