Talks aimed at ending Syria's brutal war will begin in Geneva on Friday, the UN mediator said, after a delay over who will represent the country's fractious opposition.
Negotiations had been scheduled to open on Monday, but UN envoy Staffan de Mistura told reporters in Geneva "a stalemate" over the makeup of the delegations had forced the date back.
He said he expected to be able to send invitations to the delegates on Tuesday, but declined to say who would be asked to attend, only saying the list would be "as inclusive as possible".
The announcement came as a truck suicide bomber killed at least 23 people in the Syrian city of Aleppo, most of them from the Ahrar al-Sham rebel group, according to the Syrian Observatory for Human Rights.
A rebel commander and 11 members of his family were also killed in a Russian air strike on a village between the northern province of Idlib and Aleppo, the Britain-based monitor said.
The renewed efforts to end Syria's war, which has killed more than 260,000 people and displaced half the country's population, come as the conflict approaches its fifth year.
The talks are part of a UN-backed plan agreed last year that envisages negotiations, followed by the creation of a transitional government, a new constitution and elections within 18 months.
The United States and Russia have been piling on pressure for the talks to get under way, but disagreement over who should represent the opposition has made it difficult to get started, De Mistura said.
The so-called "proximity talks" are scheduled to last six months, with the first round lasting between two and three weeks, he said, adding he expected to do "a lot of shuttling" between the sides.
"We are all feeling... the time has come to try hard to produce an outcome," De Mistura said.