BEIRUT/GENEVA/PARIS (Reuters) - Plans to hold the first negotiations to end the civil war in Syria for two years were in doubt on Wednesday after the opposition said it would not show up unless the United Nations responded to demands for a halt to attacks on civilian areas.
The Syrian government has already agreed to join the talks that U.N. envoy Staffan de Mistura hopes to convene in an indirect format in Geneva on Friday with the aim of ending the five-year-old war that has killed 250,000 people.
Washington urged Syrian opposition groups to attend.
"Factions of the opposition have an historic opportunity to go to Geneva and propose serious, practical ways to implement a ceasefire, humanitarian access and other confidence-building measures, and they should do so without preconditions," State Department spokesman Mark Toner said.
Preparations have been beset by difficulties, including a dispute over who should be invited to negotiate with President Bashar al-Assad's government as it claws back territory with help from Russia and Iran.
Kurds, who control a swathe of northern Syria, were not invited and predicted the talks would fail.
A Saudi-backed opposition council that groups armed and political opponents of Assad broke up a second day of meetings in Riyadh, saying it was waiting for a response from the United Nations to demands before it decided whether to attend.
While it has expressed support for a political solution and talks, the opposition High Negotiations Committee (HNC) says attacks on civilian areas must stop before any negotiations.
In a letter to U.N. Secretary General Ban Ki-moon, it also called for the lifting of sieges on blockaded areas among other steps outlined by the U.N. Security Council in a resolution passed last month.
"We are waiting for the response of de Mistura first, and then Secretary General Ban Ki-moon, which is the most important ... If it is positive maybe there will be an agreement to go," Asaad al-Zoubi, an HNC member, told Saudi TV channel Al Ikhbariya.
Diplomacy has so far failed to resolve a conflict that has forced millions from their homes, creating a refugee crisis in neighbouring states and Europe. With the war raging unabated, the latest diplomatic effort has been overshadowed by increased tension between Saudi Arabia and Iran.
The Syrian government, aided by Russian air strikes and allied militia including Iranian forces, is gaining ground against rebels in western Syria, this week capturing the town of Sheikh Maskin near the Jordanian border.
Russian air strikes that began on Sept. 30 have tilted the war Assad's way after major setbacks earlier in 2015 brought rebels close to coastal areas that form the heartland of Assad's Alawite sect and are of great importance to the state he leads.
While the Saudi-backed HNC includes powerful rebel factions fighting Assad in western Syria, Russia has been demanding wider participation to include Syrian Kurds who control wide areas of northern and northeastern Syria.
The Syrian Kurdish PYD party, which is affiliated with the Kurdish YPG militia, was however excluded from the invitation list in line with the wishes of Turkey, a major sponsor of the rebellion which views the PYD as a terrorist group.
The PYD's representative in France, Khaled Eissa, who had been on a list of possible delegates proposed by Russia, blamed regional and international powers, in particular Turkey, for blocking the Kurds and forecast the talks would fail.
"You can't neglect a force that controls an area three times the size of Lebanon," he said. "We will not respect any decision taken without our participation."
Russian Deputy Foreign Minister Gennady Gatilov said the PYD could join the talks at a later stage.
Haytham Manna, a prominent opposition figure allied to the PYD and invited to the talks, told Reuters he would not attend if his allies were not there.
Manna is co-leader of an opposition group called the Syrian Democratic Council (SDC), which includes the PYD and was formed in December in Kurdish-controlled Hasaka province.
Ilham Ahmed, a Kurdish politician who co-chairs the SDC, heaped criticism on de Mistura.
"We hold him responsible - not America or Russia - him and the United Nations. He was tasked with forming the delegations in a balanced way and in a way that represents all the elements of Syrian society," she told Reuters.
"When the whole of northern Syria is excluded from these negotiations, it means they are the ones dividing Syria. They are always accusing the Kurds of dividing Syria, but they are the ones dividing Syria."
Additional reporting by Mariam Karouny and John Davison, John Irish, and Omar Fahmy in Cairo; Writing by Tom Perry; Editing by Giles Elgood/Mark Heinrich