The United States and Russia threw their full weight behind long-delayed Syria peace talks that the UN said finally would be held January 22, though it was not yet clear whether key sponsors of the warring sides would attend.
"We haven't established a list yet," UN and Arab League mediator Lakhdar Brahimi said.
That left open the participation of Saudi Arabia, seen as a major sponsor the majority Sunni Muslim rebels, and Iran, which steadfastly back the regime of Syrian President Bashar al-Assad, which is dominated by Alawites derived from Islam's Shiite current.
"These two countries will certainly be among the possible participants," Brahimi said after closed-door talks in Geneva with US and Russian officials.
The talks will bring together the Syrian government and the opposition at the negotiating table for the first time since the rebellion against Assad erupted in March 2011.
UN chief Ban Ki-moon announced the date for the long-delayed conference on Syria dubbed 'Geneva II' at UN headquarters in New York.
"At long last and for the first time, the Syrian government and opposition will meet at the negotiating table instead of the battlefield," Ban told reporters.
"The fighting has raged on far too long -- with more than 100,000 dead, almost nine million driven from their homes, countless missing and detained, and terrible violations of human rights," Ban said, calling the war the "biggest threat to international peace".
The United States, which has long urged Assad to step down, called the upcoming talks the "best opportunity" to form a new transitional government to lead Syria out of war.
"In order to end the bloodshed and give the Syrian people a chance to meet their long-deferred aspirations, Syria needs new leadership," US Secretary of State John Kerry said in a statement.
He said he recognised "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."
Russia, which broadly supports the survival of Assad's regime, also underlined the importance of the talks.
On Monday, Russian President Vladimir Putin held a 35-minute audience with Pope Francis in the Vatican during which both men discussed the urgent need "to promote concrete initiatives for a peaceful solution to the conflict, favouring negotiation," the Holy See said.
They agreed any solution should involve "the various ethnic and religious groups, recognising their essential role in society," it said in a statement.
Kerry has been working with his Russian counterpart Sergei Lavrov to organize the peace talks with the UN since May.
More than 120,000 people have been killed in the conflict and millions displaced, creating a global humanitarian crisis as neighbouring states struggle to cope with the refugee tide.
Signup to our newsletter and follow us on Facebook and Twitter!
White House deputy spokesman Josh Earnest told reporters travelling with President Barack Obama aboard Air Force One that "we'll necessarily need to end (Geneva) with Assad leaving power".
Brahimi, meanwhile, said the country's warring sides should not wait until January 22 to rein in fighting.
"We very strongly appeal to the Syrian government and the opposition to not to wait for the conference," he said, urging the sides to "diminish the violence (and) release prisoners and detainees of all sorts".
He admitted though, "being realistic", that "a lot of the things that need to happen will happen after the conference starts".
The international community has long struggled to get the talks under way to follow on from a Geneva meeting held in June 2012 in world powers called for a Syrian transition government but which changed nothing.
The United States, Britain and France had to persuade the fractured Syrian opposition to set aside its demand that Assad stand down before any talks could happen.
And Russia leant on Damascus to ease up on its own condition that Assad's future must not be up for discussion.
Brahimi said the conference would start "without any preconditions".
The opposition Syrian National Coalition has agreed to attend, but its authority is not recognised by Islamist rebel groups that are proving the most effective on the battlefield.
Brahimi said the opposition coalition had a "very important role to play" and had to be "as representative as possible" for the talks to work.
"We are in touch both with the government and the opposition. We are asking them to name their delegations as early as possible, hopefully before the end of the year," said the Algerian mediator, who previously was UN envoy for Afghanistan and Iraq.
"This conference is really for the Syrians to come to Geneva and talk to one another and hopefully start a credible, workable effective peace process for their country."
Iran's mooted presence at the talks, pushed by Russia, long raised hackles among Western powers, which pushed Saudi Arabia as a counterweight participant.
But last weekend's breakthrough interim deal on Iran's nuclear programme signed with the P5+1 group of the United States, Russia, France, Britain, Germany and China has opened up the prospect of Tehran officials being more welcome.
During those Iran/P5+1 negotiations, also held in Geneva, Brahimi met Iranian Foreign Minister Mohammad Javad Zarif, Lavrov, and top US negotiator Wendy Sherman.
Brahimi said he would meet US and Russian officials again on December 20.