HANOI: Four men were instantly killed when a bomb left over from the Vietnam War blew up as they were trying to open it to remove explosive material, police said Wednesday.

The dead were aged between 24 and 27 and were two pairs of brothers, said Nguyen Van Dieu, a police official in southern Tay Ninh province.

"They were killed on the spot. District police are investigating what sort of bomb it was. The accident area was a target of US attacks in the war," the official from Don Thuan commune told AFP of Tuesday's explosion.

Over 10,500 people have been killed in Vietnam's central provinces by bombs left over from the war, which ended in 1975, according to an earlier report by the Vietnam Veterans of America Foundation and Vietnam's Ministry of Defence.