The gathering in Astana, the Kazakh capital, is also the start of a new effort to end six years of carnage that has killed hundreds of thousands, displaced half of Syria's population and sent millions of refugees to neighboring countries and Europe.
The UN envoy to Syria, Staffan de Mistura, is participating in the talks, which if successful, are expected to be followed by more political talks in February in Geneva.
Osama Abo Zayd, a rebel media representative to the talks, told The Associated Press before the start that the scope of the negotiations is limited to strengthening the cease-fire.
"There's no significance to negotiations if the people on whose behalf we are negotiating are being killed," he said, adding that there has been absolutely no discussion about elections or Assad's future.
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Ahead of the talks, delegates passed through the hotel's soaring atrium, where songbirds are kept to chirp in cages, on their way to the conference room.
Reflecting persisting tensions, Arab TV stations said the rebel delegates stalled, entering the room a few minutes late to register their displeasure at being seated at the same oval-shaped table as the Iranian delegation. The hotel was closed off to all but a handful of representatives of the media.
After a short opening ceremony during which Kazakh Foreign Minister Kairat Abdrakhmanov spoke, the meeting went into closed session. It wasn't immediately clear if there would be any direct talks between the rebels and Damascus representatives behind the closed doors.