The latest round of peace talks between Syria's government and opposition are set to wrap up Friday, with no progress on ending a three-year civil war.
Both sides continue to disagree over even what to discuss at the Geneva talks that began in January, and it is not clear whether a third round will be held.
United Nations mediator Lakhdar Brahimi said Thursday after meeting diplomats from Russia and the U.S. that the talks are "not making much progress."
"Failure is always staring at us in the face. As far as the United Nations is concerned, we will certainly not leave one stone unturned if there is a possibility to move forward."
The opposition earlier this week proposed a transitional government, in a plan that avoided any mention of Syrian President Bashar al-Assad.
Many viewed the move as a concession, since the opposition has insisted it would be satisfied with nothing short of President Assad's removal.
But Damascus refused to even discuss the plan, saying it was a distraction from discussing combating terrorism - what it views as the talks' main purpose.
Brahimi is expected to hold only separate meetings with Syria's government and rebel groups on Friday.
Also Friday, the U.N. Security Council continues discussions on a draft resolution that, among other things, expresses "grave alarm" over civilians trapped by Syrian forces.
Russia, Syria's main ally, is threatening a veto, calling it too one-sided against the Syrian government.
U.N. emergency relief coordinator Valerie Amos said Thursday getting aid to desperate civilians in Syria should not be a matter of controversy or politics.
Amos briefed the Security Council on what she says is modest progress in getting help to civilians This includes evacuating more than 1,400 people from the city of Homs.
A brief "humanitarian pause" last week gave women, children, and old men a chance to get out.
Amos said the Homs agreement shows what can be done. But she said it cannot be a model because aid workers came under fire.