A fourth round of talks in Kazakhstan’s capital Astana began on Wednesday to find a way to reinforce a cease-fire put into place at the end of last year but which has been widely violated.
The negotiations are sponsored by Turkey, Russia and Iran.
The source, who spoke on condition of anonymity due to restrictions on talking to the media, said the opposition was withdrawing following Russian attacks in Hama and Deraa provinces.
Before abandoning the talks, the opposition delegation submitted proposals for ending the fighting, the source added.
The Astana talks, which first convened in January, are running alongside UN-backed discussions being held in Geneva.
Turkey, which supports opponents of Bashar al-Assad’s regime, joined Assad allies Russia and Iran in sponsoring the negotiations after the assault on Aleppo in December.
Syria has been locked in civil war since March 2011. According to the UN envoy to Syria, around 400,000 people have died in the conflict while half the population has been driven from their homes.