BEIRUT (AP) — Syrian opposition activists say an explosion in the country's north has killed at least 18 people and wounded many others. The cause of the blast wasn't immediately known.

The opposition-run Syrian Civil Defense, also known as White Helmets, says the blast occurred Sunday in the village of Sarmada near the Turkish border and "killed and wounded dozens." The explosion collapsed a five-story building, burying many of the victims.

The Britain-based Syrian Observatory for Human Rights put the death toll at 18. It's unclear if the casualties are all civilians.

The Observatory says an arms depot in the basement of a building had detonated. It says the depot was run by an arms dealer close to the al-Qaida-linked Levant Liberation Committee.

Meanwhile, Syrian government forces are fighting rebels in Idlib province.