About a dozen children were among 39 civilians killed in a blast at a weapons depot that destroyed buildings in an insurgent-held town in northwestern Syria, according to the Syrian Observatory for Human Rights monitoring group.

The blast of unknown origin caused the collapse of two buildings in Sarmada in Idlib province near the Turkish border, Agence France-Presse reported. Rescuers removed rubble with bulldozers and extracted bodies and five trapped survivors.

Three members of the al-Qaeda-linked Hayat Tahrir al-Sham (HTS) militant group were also killed, said Rami Abdel Rahman, head of the UK-based monitoring group.

“The explosion occurred in a weapons depot in a residential building in Sarmada,” Rahman said. However, the cause of the blast was “not yet clear.”