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."