BEIRUT/GENEVA (Reuters) – The United Nations said on Monday it would issue invitations for marathon Syrian peace talks to begin this week, but opposition groups signaled they would stay away unless the government and its Russian allies halt air strikes and lift sieges on towns.

The first talks in two years to end the Syrian civil war were meant to begin on Monday but have been held up in part by a dispute over who should represent the opposition to President Bashar al-Assad. U.N. envoy Staffan de Mistura said he was still working on his list, and expected to issue the invitations on Tuesday for talks to start on Friday.

The aim would be six months of talks, first seeking a ceasefire, later working toward a political settlement to a war that has killed more than 250,000 people, driven more than 10 million from their homes and drawn in global powers.

The ceasefire would cover the whole country except parts held by Islamic State militants and al Qaeda’s Syrian branch, the Nusra Front, de Mistura told a news conference in Geneva.

De Mistura, whose two predecessors quit in apparent frustration after holding failed peace conferences of their own, acknowledged the going would be difficult. Delegations would meet in separate rooms in “proximity talks”, with diplomats shuttling between them. Threats to pull out should be expected.

“Don’t be surprised: there will be a lot of posturing, a lot of walk-outs or walk-ins because a bomb has fallen or someone has done an attack…. You should neither be depressed nor impressed, but it’s likely to happen,” he said. “The important thing is to keep momentum.”

The spokesman for one of the rebel groups in the opposition High Negotiating Committee (HNC) said it was impossible for the opposition to attend as long as rebel territory is being pounded by air strikes and besieged towns are being starved.

“It is impossible to give up any of our demands. If we attend, it’s as if we are selling our martyrs,” said Abu Ghiath al-Shami, spokesman for Alwiyat Seif al-Sham, one of the groups fighting against Assad’s forces in the southwest.

U.S. Secretary of State John Kerry said he expected clarity within a day or two over who would attend, and expressed support for de Mistura’s decision to take time to draw up the list.

“We don’t want to decide and have it crumble on day one. It’s worth taking a day or two, or three, or whatever,” Kerry said during a visit to Laos.

The outcome was up to the Syrian parties, he added: “They have to be serious. If they are not serious, war will continue. Up to them. You can lead a horse to water; you can’t make it drink.”

RUSSIAN INTERVENTION

Years of high-level diplomacy have so far yielded no progress toward ending or even curbing the fighting. Since the last peace conference was held in early 2014, Islamic State fighters have declared a caliphate across much of Syria and Iraq, and the war has drawn in world powers.

The United States has led air strikes against the militants since 2014, and Russia launched a separate air campaign nearly four months ago against enemies of its ally Assad.

Russian firepower has helped the Syrian military and its allies achieve military gains, including a major push in the northwest of the country in recent days, with rebels acknowledging a turn in momentum.

The rise of Islamic State and Russia’s entry into the war have given new impetus to diplomacy, leading to a Dec. 18 U.N. Security Council resolution, backed by Washington and Moscow, that called for peace talks.

But world powers remain at odds over who should be invited. Russia says opposition figures it calls terrorists must be excluded, and wants to include groups like the Kurds who control wide areas of northern Syria. Regional heavyweight Turkey opposes inviting the Kurds.

The main Sunni Arab opposition groups, who are supported by Arab governments and the West, say they will not attend unless they can choose their own delegation. Spokesman Salim al-Muslat said the opposition HNC would discuss its position on Tuesday.

The HNC, formed in Saudi Arabia last month and grouping armed and political opponents of Assad, has repeatedly said talks cannot begin until air strikes are halted, government sieges of rebel-held territory lifted and detainees freed, steps outlined in the U.N. resolution.

“Unfortunately, it is not possible to sit and talk to anyone without the suffering being lifted first,” Muslat said on Arabic news channel Arabiya al-Hadath.

SUICIDE BOMBING

The peace conference, if it takes place, will be the third since the war began and the first convened by de Mistura, a veteran diplomat with dual Swedish and Italian nationality.

All previous diplomatic efforts foundered over the future role of Assad, with the opposition refusing to back off its demand that he leave power and the president refusing to go.

A suicide bomber driving a fuel tank truck blew himself up at a checkpoint run by the Islamist rebel group Ahrar al-Sham in the northern city of Aleppo on Monday, killing at least 23 people, the Syrian Observatory for Human Rights monitoring group said.

Rebel forces lost the town of Rabiya in Latakia province over the weekend, their second major setback near the Turkish border in northwestern Syria in recent weeks as the government and Russia seek to cut rebel supply lines to Turkey.

“It’s a major pullback for us, but it is not over. We have 17 martyrs and 30 wounded. And none of the injuries are from bullets: it is all due to shrapnel from missiles, proof of how we are struggling to fend off Russian air strikes,” Firas Pasa, a leader of an ethnic Turkmen rebel group told Reuters in Gaziantep, Turkey, near the border.

“If the West wants us to defeat Syrian government forces then we urgently need anti-aircraft capabilities,” he said.

A member of Ahrar al-Sham, Abu Baraa al-Lathkani, said the rebels had abandoned the strategy of trying to hold territory and were shifting to guerrilla tactics.

The Syrian military, its morale running high, is planning the next phase of its offensive in northern Syria. The coming target is Idlib, a rebel stronghold, said a military source.

(Additional reporting by Lisa Barrington and Tom Perry in Beirut, Suleiman Al-Khalidi in Amman, David Brunnstrom in Vientiane, Humeyra Pamuk in Turkey and Stephanie Nebehay in Geneva; Writing by Peter Graff; Editing by Mark Trevelyan)