By Tom Miles and John Irish
Syria's main opposition group has met UN mediator Staffan de Mistura for the first time, but the talks ran straight into trouble after Islamic State bombers killed more than 60 people near the country's holiest Shi'ite shrine.
Representatives of the Saudi-backed Higher Negotiation Committee (HNC) - which includes political and militant opponents of President Bashar al-Assad - warned they may yet walk away from the Geneva talks unless the suffering of civilians in the five-year conflict is eased.
The head of the Syrian government delegation retorted that the blasts in Damascus, which the Interior Ministry blamed on a car bomb and two suicide bombers, merely confirmed the link between the opposition and terrorism - even though Islamic State has been excluded from the talks.
The United Nations is aiming for six months of negotiations, first seeking a ceasefire, later working toward a political settlement to the civil war that has also killed over 250,000 people and driven more than 10 million from their homes.
Only on Friday, the HNC said it would boycott the process, insisting it wanted an end to air strikes and sieges of Syrian towns before joining the negotiations.
This forced de Mistura - who invited the government and opposition umbrella group for "proximity talks", in which he would meet each side in separate rooms - to set the ball rolling with only the government delegation.
Under intense pressure, notably from the US, the HNC later relented and arrived in Geneva on Saturday. However, the group questioned how long the delegation would stay.
"In view of the (Syrian) regime and its allies' insistence in violating the rights of the Syrian people, the presence of the HNC delegation in Geneva would not have any justification and the HNC could pull its negotiating team out," the group's coordinator, Riad Hijab, said in a statement.
UN Secretary-General Ban Ki-moon described the talks as long overdue.
"I urge all parties to put the people of Syria at the heart of their discussions, and above partisan interests," he said.
A spokeswoman for de Mistura said the UN mediator had met the opposition delegation at its hotel, while his deputy Ramzi Ezzedine Ramzi visited the government delegates at theirs. The talks will continue on Monday.
In Washington, US Secretary of State John Kerry urged both sides to seize the opportunity to make progress.
"In the end there is no military solution to the conflict," he said in a televised statement.
However, opposition delegate Bassma Kodmani denied that her side was ready yet to negotiate.
"We only came to Geneva after receiving assurances and commitments ... that there would be serious progress on the humanitarian situation," she told a news conference.
"We can't start political negotiations until we have those gestures."
The Syrian government's delegation head in Geneva, Bashar al-Jaafari, said the government was considering moves such as the creation of humanitarian corridors, ceasefires and prisoner releases, but suggested they might come about as a result of the talks, not before them.
"Absolutely, this is part of the agenda that we agreed upon and that will be one of the very important topics we will discuss among ourselves as Syrian citizens," Jaafari said.
Islamic State claimed responsibility for Sunday's attacks in the Sayeda Zeinab district of Damascus, according to Amaq, a news agency that supports the militant group. It said two operations "hit the most important stronghold of Shi'ite militias in Damascus".
The Britain-based Observatory put the death toll at over 60, including 25 Shi'ite fighters.