BEIRUT (AP) — Four children were killed Monday when a mine and explosives left inside an abandoned apartment went off in a rebel-held town in northern Syria, an opposition war monitor and a paramedic group said.

The opposition’s Syrian Civil Defense, also known as White Helmets, said the brothers were killed instantly by the blasts in the town of Binnish, and their bodies were taken a nearby morgue. It gave no further details.

The Britain-based Syrian Observatory for Human Rights also reported that four siblings were killed, adding that the building where the explosives went off had been used for years by families displaced by Syria’s 11-year civil war.

The Observatory, which documents Syria’s conflict, said 176 people, including 10 women and 91 children, have been killed by unexploded ordnance throughout Syria this year.

The conflict that began in March 2011 has killed hundreds of thousands of people and displaced half of Syria’s pre-war population of 23 million. It is not uncommon for booby-traps and mines to explode and kill and maim people in different parts of Syria.

Over the past few years, Syrian President Bashar Assad’s forces have gained control of much of the country with the help of his main backers Russia and Iran.

Binnish is in the last remaining rebel stronghold in northwest Syria.