The first full day of peace talks between the Syrian government and opposition is due to begin in Geneva.
Syria's warring parties are due to begin formal peace talks in Geneva, for the first time since the conflict began three years ago.
On Thursday UN mediator Lakhdar Brahimi shuttled between the two sides to assess their willingness to meet together.
The government and rebel delegations will be in separate rooms and conduct negotiations through a UN mediator.
The opening day of the conference was hosted in the Swiss city of Montreux and saw bitter exchanges between the rival delegations.
There are still entrenched divisions - with the government wanting to focus on terrorism, and the opposition on how to remove President Bashar al-Assad.
The three-year conflict has claimed well over 100,000 lives, the UN says.
Diplomats say a main concern will be ensuring neither side walks out.
Syria's civil conflict has claimed well over 100,000 lives, the UN says.
The violence has also driven 9.5 million people from their homes, creating a major humanitarian crisis within Syria and for its neighbours.
The violence has also driven 9.5 million people from their homes, creating a major humanitarian crisis within Syria and for its neighbours.
On Thursday Mr Brahimi held behind-the-scenes negotiations between representatives of President Bashar al-Assad's government and of the National Coalition.
Unprecedented talks
The talks are unprecedented seeing as the two sides have never before agreed to hold formal discussions, the BBC's Bridget Kendall reports from Geneva.
This is the third day of the conference, but the first in which negotiations get under way in earnest, after UN mediator Lakhdar Brahimi shuttled between the two sides on Thursday assessing their willingness to meet.
Western diplomats say Mr Brahimi's main concern for Friday may be to ensure that neither delegation stage a walkout, our correspondent reports.
The opening meeting on Wednesday in the Swiss resort of Montreux saw acrimonious exchanges.
One small encouraging sign for Friday's talks is that both sides will be in the same room for the opening session on Friday, she adds.
The Geneva talks are unprecedented, as the two sides have never before agreed to hold formal discussions.
However, they are not expected to talk to each other - after being addressed jointly by Mr Brahimi, they will consult in separate rooms to work out initial positions.
The talks are ostensibly about the implementation of the Geneva I communique, which was agreed at a previous summit in 2012 and calls for a transitional government in Syria with full executive powers.
A schedule for the duration of the rest of the talks is due to be set on Friday, but even setting the agenda may be problematic.
But the views of the government and the opposition are so diametrically opposed that Friday's discussions may get no further than preliminary attempts to set a common agenda, the BBC's Bridget Kendall in Geneva reports.
The government is expected to insist on the importance of fighting "terrorism", while the opposition will want the removal of Mr Assad high on the agenda.
And even that may prove difficult: The government is expected to insist on the importance of fighting what it sees as terrorism, while the opposition wants a high priority to be put on the removal of Mr Assad.
'Super-magnet for terrorism'
One small encouraging sign for Friday's talks is that both sides will be in the same room for the opening session, our correspondent adds.
The head of the National Coalition, Ahmed Jarba, told reporters on Thursday: "I think that the world is convinced today that Assad is not staying and will not stay in power."
However, they are not expected to talk to each other - and after being addressed jointly by Mr Brahimi, they will go to separate rooms to work out their initial positions.
"We have begun to look towards the future, the future of a free and prosperous Syria," he added.
But analysts are hopeful that some progress can be made.
The removal of Mr Assad is a key opposition demand and was echoed by US Secretary of State John Kerry, calling Mr Assad "a one-man super-magnet for terrorism" in an interview with the Al Arabiya TV channel.
While the two sides are diametrically opposed on many issues, they have both indicated a willingness to talk about concrete steps like local ceasefires, prisoner exchanges and establishing safe corridors for the delivery of badly-needed humanitarian aid.
Syrian officials have flatly rejected any suggestion of Mr Assad stepping down and he has suggested he will run for president again in elections due this year.
Assad's role
A senior Russian official has told the BBC that nobody other than President Assad could run Syria at the moment.
One of the main sticking points between the government and the rebels is the role of Bashar al-Assad.
Deputy Prime Minister Arkady Dvorkovich said the conflict was a battle for power and not a fight for democracy.
The opposition demands his removal from office as a condition for peace.
He said the toppling of the Libyan and Egyptian leaders had ended in disasters and Russia wanted stability.
It is supported in this by many key foreign observers: The US Secretary of State John Kerry has called Mr Assad "a one-man super-magnet for terrorism".
The Geneva I communique, agreed at a previous summit in 2012, calls for a transitional government in Syria with full executive powers.
But Syrian officials have flatly rejected any suggestion of Mr Assad stepping down, and he has even suggested he will run for president again in elections due this year.
With no movement likely from either side on the bigger political questions, Mr Brahimi has suggested both sides may be willing to discuss more short-term measures such as local cease-fires, prisoner exchanges and establishing safe corridors for the delivery of badly-needed aid.
The Syrian government also has its supporters: Russia's Deputy Prime Minister Arkady Dvorkovich has told the BBC that nobody other than Mr Assad can run Syria at the moment.
But correspondents say that with the lack of trust on either side, even that may collapse into disagreements, and it remains to be seen how much influence the National Coalition can wield over the full spectrum of opposition groups, some of whom have rejected the Geneva process.
Iran, Syria's main ally in the region, has been excluded from the talks in Switzerland, despite earlier being invited by UN Secretary General Ban Ki-moon.
Speaking at the World Economic Forum in Davos on Thursday, Iranian President Hassan Rouhani called for the Syrian opposition and government to sit round a table as the prelude to "free and fair elections".
It was for the Syrian people to decide their future, he added.
"No outside party or power can decide for the Syrian people and Syria as a country. We all have to help the people."
Meanwhile, the head of al-Qaeda Ayman al-Zawahiri called on Thursday on rebel groups in northern Syria to stop fighting one another.
Clashes between rebels and the al-Qaeda-affiliated Islamic State of Iraq and the Levant (Isis) have been going on for three weeks. The UK-based Syrian Observatory for Human Rights said 1,400 people have died in the violence.
In an internet audio message, Mr al-Zawahiri called on "every free person in Syria seeking to overthrow Assad... to seek an end to fighting between brothers in jihad and Islam immediately".
