Syria conflict: Geneva peace talks set for January
UN Secretary General Ban Ki-moon has said Syria's government and opposition will for the first time attend peace talks - in Geneva on 22 January.
It would be "unforgivable not to seize this opportunity to bring an end to the suffering and destruction", he warned.
The UN, US and Russia have been trying for months to get both sides to agree a political solution to the conflict.
But there have been disputes over who should represent the opposition and the fate of President Bashar al-Assad.
The UN special envoy to Syria, Lakhdar Brahimi, said he hoped Syrian parties to the talks would name their delegations by New Year.
He said a full list of participants was yet to be established, adding that Iran and Saudi Arabia were "possible participants".
Hopes of a breakthrough in Geneva were raised after world powers agreed a deal with Iran on Sunday over its controversial nuclear programme.
In a statement, a spokesperson for Mr Ban expressed deep appreciation for the work of US and Russian diplomats, as well as Mr Brahimi.
"We will go to Geneva with a mission of hope," the statement said.
Speaking to reporters later on Monday, Mr Ban himself stressed that the Geneva conference "was the vehicle for a peaceful transition" in Syria.
Mr Ban said he expected representatives of both sides to come "with a clear understanding" that the goal of the talks was the full implementation of the Geneva Communique, issued after a meeting of the UN-backed Action Group for Syria in the Swiss city in June 2012.
He said an "inclusive, Syria-led process" was "the only way to end violence" in the country.
And Mr Ban reiterated that the peace talks would seek to establish a transitional government with full executive powers - as envisaged in the Geneva Communique.
The document says such a cabinet should also have the authority over Syria's military and security forces, that could include officials serving under Mr Assad and members of the opposition.
Mr Ban warned all parties that they should also come to Geneva "with a serious intention to end a war that has already left well over 100,000 dead, driven almost nine million from their homes, left countless missing and detained, sent tremors through the region and forced unacceptable burdens on Syria's neighbours".
"The conflict in Syria has raged for too long. It would be unforgivable not to seize this opportunity to bring an end to the suffering and destruction it has caused."
It was not clear from the statement if Iran would be invited to Geneva, but Mr Ban said he expected "all regional and international partners to demonstrate their meaningful support for constructive negotiations".
Meanwhile, Russian Deputy Foreign Minister Gennady Gatilov said Iran should take part in the Geneva talks, but there was no final agreement on that issue, Russia's Interfax news agency reports.
A spokesman for the Supreme Military Council of the Free Syrian Army, Luay Mekdad, told the BBC that the international community had not as yet created the right atmosphere for the Geneva talks to go ahead.
Earlier this month, the main opposition alliance, the National Coalition, agreed to attend what has become known as the "Geneva II" conference if a number of conditions were met.
Relief agencies would have to be given access to besieged areas and all detainees, particularly women and children, had to be freed, it said. The coalition's leader, Ahmed Jarba, also stressed that the president would "have no role in the transitional period".
The government has said it will "in principle" attend the conference.
However, it has also said it will not negotiate with "terrorists", while dismissing almost all those who oppose it as such. It has also insisted repeatedly that any political solution will not involve Mr Assad's departure.
The BBC's Jim Muir in Beirut says Mr Assad is feeling in a stronger position than he has for a long time and shows no intention of giving up.
In the past month, several towns around Damascus and the second city of Aleppo have been recaptured by soldiers reportedly backed by pro-government militiamen, members of the Lebanese Shia Islamist movement Hezbollah and Iranian Revolutionary Guards.
On Monday, there were reports of heavy casualties on both sides from clashes between government forces and rebels outside Damascus.
Rebel fighters, including those from the newly formed Islamic Front, have been trying for the past three days to end the siege of opposition-held areas in the Eastern Ghouta, an agricultural belt around the capital, which has prevented the delivery of humanitarian supplies.
The Syrian Observatory for Human Rights reported that several villages and checkpoints had been captured as the rebels tried to secure roads.
There has been no word from state media, but the UK-based activist group said both sides had suffered losses, with at least 160 combatants killed over the weekend, including 100 rebels and 20 members of a pro-government Iraqi Shia militia, the Abu al-Fadl al-Abbas Brigade.