A war-era rocket head uncovered in a newly-cleared forest exploded on Thursday, killing a 13-year-old boy and severely injuring three other children in Battambang’s Samlot district, a hotspot of fighting during Cambodia's civil war.

The children found the head of a Chinese-made 107mm rocket near their home in Kampong Lpao commune's O'chorm Kandal village, said Keo Muk, the Kampong Lpao commune police chief.

“They did not know it was a weapon and played with it when their parents were out for work,” he said.

The oldest boy, Sam Bros, hit the rocket head with a stone, detonating it at about 4:30 pm, killing him instantly, Muk said. He said the explosion seriously injured the boy's 5-year-old younger brother Sam Thab; a 4-year-old girl, Pheak Sreypich; and a 10-year-old boy, Tith Van.

He said the victims just recently moved from nearby Pursat province to live in the village, so they were unaware of the dangers of unexploded ordnance in the area.

The younger children received leg injuries and were admitted to the hospital, he said.

In a statement on Facebook, Cambodian Mine Action Centre head Heng Ratana said the area had seen heavy fighting during the war.

"It's vital to recall that CMAC has cleared a number of minefields in this commune and found many anti-tank landmines with double or triple in each hole," Ratana said.

Although the war is over, devastation "by remnants of war does not end yet", he said.