Ban Ki-moon, the UN secretary-general, said the goal was to agree a mutually acceptable transitional administration as well as the other elements of an outline peace plan drafted by the Western powers and Russia at Geneva in June last year.
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"It is a huge opportunity for peace that shouldn't be wasted," Lakhdar Brahimi, Ban's special envoy for Syria, told a news conference in the Swiss city, where the long delayed face-to-face talks should now take place in eight weeks' time.
Syrians and diplomats have few illusions about how hard it will be to end a civil war that has killed over 100,000 people since 2011, driven over a third of the population from their homes and divided the country among rival and often religiously driven factions with an array of competing foreign sponsors.
But a day after Assad's regional ally Iran cut a deal on its nuclear program with the United States and other world powers to ease fears of a wider war in the Middle East, UN officials spoke of a chance to start staunching the bloodshed.
It remained unclear whether Iran would attend – nor is it clear who will represent the divided Syrian opposition.
"There are many challenges ahead and no one should underestimate the difficulties," said a spokesman for US President Barack Obama as he welcomed a date for the talks.
"The United States has long made clear that there is no military solution to the violence in Syria," he added.
Russia, a vital supplier to Syria which has shielded Assad from Western demands for UN sanctions and from rebel demands that he step down before negotiations can start, again blamed the opposition for holding up the peace conference.
"It could have been held much earlier if the opposition had felt responsibility for its country and had not put forward preconditions when we met in September, October, November," Foreign Minister Sergei Lavrov was quoted as saying by state-run Russian news agency RIA.
'Eyes wide open'
US Secretary of State John Kerry, fresh from a weekend working with Lavrov on the Iran nuclear deal at Geneva, said in a statement: "We are well aware that the obstacles on the road to a political solution are many, and we will enter the Geneva conference on Syria with our eyes wide open."
After so much blood has been spilled since demonstrations for democracy began during the Arab Spring, many on either side of Syria's divide see only outright victory guaranteeing their own survival. But neither has been able to deliver a knockout blow, giving mediators a chance to argue for compromise.
The deep engagement of neighboring powers in the conflict, notably Shiite Iran behind Assad and Sunni Saudi Arabia behind the rebels, has also complicated efforts to defuse it.
Kerry and Brahimi said the presence of Iranian officials at the Syria conference - something Moscow supports and Washington has so far opposed - was yet to be decided.
"We will continue to work in concert with the UN and our partners on remaining issues, including which countries will be invited to attend and what the agenda will be," Kerry said.
For Western governments, Iran's reluctance to endorse last year's international accord on Syria has been a bar to its attendance at talks widely referred to as "Geneva 2".
Ban described the aim of the new summit as the "full implementation of the Geneva communiqué of 30 June, 2012".
A spokeswoman for the British Foreign Office said: "Until Iran publicly endorses the Geneva communiqué, and therefore makes clear that it supports the purpose of the Geneva 2 conference, it is hard to see how it can play a constructive role in finding a political solution to the conflict."
However, one senior European Union diplomat said that after Sunday's deal with Tehran: "I cannot imagine Washington continuing to object to an Iranian presence."
Brahimi urged the warring parties to try and start taking the heat out of the conflict, for example by freeing prisoners. But asked whether he aimed for a ceasefire for the start of the talks, he said: "Being realistic, a lot of the things that need to happen will happen after the conference starts, not before."
He said he hoped both Syrian sides would name delegations before the end of the year. Assad, battling to extend a ruling dynasty established by his late father four decades ago, is expected to dispatch trusted aides to speak for him.
For the opposition, the task is complicated by disputes among rebels fighting on the ground, including hardline Islamist, and exile politicians backed by Western powers.
On Sunday, Brahimi met leaders of the Syrian National Coalition, the umbrella body which has been disowned by many rebel commanders. The UN envoy said on Monday that the SNC would play an important role in forming the delegation.
"But," he added, "I have always said that the delegation has to be credible and as representative as possible.
"This conference is really for the Syrians to come to Geneva to talk to one another and hopefully start a credible, workable, effective peace process for their country."
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