Rescue workers used a bulldozer to remove rubble and extract trapped people, the correspondent said.
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 jihadists from Syria's former Al-Qaeda affiliate, who had been displaced to the area from the central province of Homs.
Behind mounds of rubble, the facade of a building was scorched black, due to a fire after the blast.
A civil defence source told AFP that women and children were among the dead.
But rescue workers had pulled out "five people who were still alive", the source said.
Most of Idlib is controlled by rebels and Hayat Tahrir al-Sham, but the Islamic State group also has sleeper cells in the area.
The regime holds a small slither of southeastern Idlib.
In recent months, a series of explosions and assassinations -- mainly targeting rebel officials and fighters -- have rocked the province.
While some attacks have been claimed by IS, most are the result of infighting since last year between other groups.
In recent days, regime forces have ramped up their deadly bombardment of southern Idlib and sent reinforcements to nearby areas they control.
Around 2.5 million people live in the province, half of them displaced by fighting in other parts of the country.
More than 350,000 people have been killed and millions displaced since Syria's civil war started in 2011 with the brutal repression of anti-government protests.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)