Syrian White Helmet civil defence workers extinguishing a fire at the scene of an explosion that hit a five-story building, in the village of Sarmada, near the Turkish border. Picture: Syrian Civil Defense White Helmets via AP

Damascus - The death toll in a blast at an ammunition depot in the rebel Syrian stronghold of Idlib province has reached 69, a monitoring group said Monday.

Seventeen children were among those killed, the Britain-based Syrian Observatory for Human Rights said.

Abdel Rahman Hassan, from the civil rescue team White Helmets, told dpa that the group had documented 67 deaths.

"We have also rescued 17 people alive, who were under the rubble," Hassan said.

The explosion took place on Sunday inside an ammunition depot in a residential building in the town of Sarmada in the north-western province of Idlib, the Observatory said.

The blast caused the multi-storey building to collapse, local activists said. 

Idlib is largely controlled by Hayat Tahrir al-Sham, an Islamist alliance led by an al-Qaeda-linked group.

The Observatory said among the dead were at least three members of Hayat Tahrir al-Sham.

During the last three years, thousands of rebels and their families have been evacuated from different parts of war-torn Syria to Idlib under agreements with the government.