"Our delegation's meetings are over for today," a source close to the regime told AFP, after rebel spokesman Yehya al-Aridi told reporters the opposition's meetings were also done for the day.
Today's talks, organised by rebel backer Turkey and regime allies Russia and Iran, could have marked the first time armed rebel groups directly negotiate with Assad's regime since the conflict began in 2011.
But the rebels backed out of direct talks because of the regime's ongoing bombardment and attacks near Damascus.
Aridi told reporters the "two sides are working on issues relating to the confirmation of the ceasefire" brokered last month by Ankara and Moscow.
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He said that the rebel delegation had met with Russia -- which launched a bombing campaign in support of Assad in September 2015 -- and discussed "achieving a ceasefire in Syria".
The talks are scheduled to continue on Tuesday, with Aridi saying he expected they would take place "through mediation."
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