Syria Geneva II: First full day of talks begin
A UN negotiator is meeting members of both Syria's government and the opposition on Friday, in an effort to end the three-year civil conflict.
But despite hopes of a joint meeting between the two sides, mediator Lakhdar Brahimi is holding talks with each in a different room at a different time.
Both sides blame the other for the separate meetings.
The Damascus delegation has complained about defiant remarks made by the opposition chief Ahmed Jarba.
On Thursday Mr Jarba called President Assad and his regime a "political corpse" which could not be part of Syria's future.
For its part, the opposition has said it will not meet government delegates face-to-face until they have signed a written commitment accepting the Geneva communique drafted 18 months ago, which calls for a transitional government.
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.
This is the third day of the conference, but the first in which negotiations get under way in earnest.
Mr Brahimi is meeting representatives for the Assad regime in the morning, and will hold talks with opposition delegates later on Friday.
The talks are ostensibly about the implementation of the Geneva I communique, 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.
She says 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. And now the two sides are no longer going to meet face-to-face at the beginning of the discussions - and perhaps throughout the whole conference.
Diplomats say the setback is not surprising and is not a sign of the talks collapsing.
And analysts are still hopeful that at least some progress can be made.
While the two sides are 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.
One of the main sticking points between the government and the rebels is the role of Bashar al-Assad.
The opposition demands his removal from office as a condition for peace.
It is supported in this by many key foreign observers: US Secretary of State John Kerry has called Mr Assad "a one-man super-magnet for terrorism".
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.
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.