The ambush was also reported by Syrian state television, which said "tens of terrorists" had been killed in the attack in the west of the province.
The Syrian Observatory for Human Rights, a Britain-based NGO, said another 26 rebels were wounded in the ambush and 12 more were missing.
Both the Observatory and state media said the attack took place between the towns of Hara and Zamreen in western Daraa.
Hara is under the control of regime forces, but Zamreen is under rebel control like much of the surrounding area, although there have been recurring clashes there in recent weeks.