But the cause of the blast was “not yet clear”, Abdel Rahman added.
He said most of those killed were family members of fighters from Hayat Tahrir al-Sham, an alliance led by militants from Syria 's former Al-Qaeda affiliate, who had been displaced to the area from the central province of Homs.
In recent days, regime forces have ramped up their deadly bombardment of southern Idlib and sent reinforcements to nearby areas they control.
President Bashar al-Assad has warned that government forces intend to retake Idlib, after his Russia-backed regime regained control of swathes of rebel-held territory elsewhere.
Around 2.5 million people live in the province, half of them displaced by fighting in other parts of the country.