A 60 mm mortar shell exploded in northern Cambodia Thursday afternoon, claiming the livers of two boys and a girl, the local police said.

The victims were aged between 8 and 13 years old, said Pheng Lun, police chief of the rural Por commune, where the accident took place.

"The victims found the mortar shell while they tended cattle at rice fields, and they used rocks to beat on it, triggering the explosion," he said, adding that two of the victims were siblings.

The mortar round was left during the war time, he said.

Landmines and unexploded ordnance killed 22 people and injured 89 others in 2013, according to a report of the Cambodian Mine Action Authority released early this week.

The Southeast Asian nation is one of the world's worst countries suffering from landmines and unexploded ordnance. An estimated 4 million to 6 million landmines and other munitions were left over from three decades of war and internal conflicts that ended in 1998.

Heng Ratana, director general of the Cambodia Mine Action Center, said that more than 3 million landmines and unexploded ordnance have been removed and destroyed, and the country is seeking about 50 million U.S. dollars a year until 2020 to entirely get rid of all types of anti-personnel mines.