BEIRUT, Lebanon — The death toll from an explosion in a residential building thought to store weapons and ammunition in Syria’s rebel-held Idlib Province has risen to 69, including 12 children, a monitoring group said on Monday.

The explosion on Sunday happened in the town of Sarmada, which is near the border with Turkey and north of the provincial capital, Idlib, said the group, the Syrian Observatory for Human Rights, which monitors the conflict from Britain with the aid of a network of contacts in Syria.

Syrian civilians, displaced during the country’s seven-year war, were living in the building, which was also used by an arms dealer, the group said.

At least 52 civilians were killed, the Syrian Observatory for Human Rights reported, along with 17 members of Hayat Tahrir al-Sham, a jihadist-led alliance.