PHNOM PENH, Cambodia (AP) — Four American demining experts were wounded Tuesday by an accidental explosion at Cambodia's main training center for the removal of unexploded ordnance, its director said.

Heng Ratana, the director general of the Cambodian Mine Action Center, said the accident occurred during a break in training sessions in the afternoon in the central province of Kampong Chhnang.

He said the four victims, two of them critically injured, were evacuated by helicopter to a hospital in the capital, Phnom Penh. The names of the men, who Heng Ratana said arrived in Cambodia last week, have not been released.

"Four U.S. personnel were injured today in Kampong Chhnang during a mine clearance training exercise," U.S. Embassy spokesman John Simmons confirmed.

While details of the accident were not clear, Meas Chamrouen, a trainee at the center, said machine gun ammunition had exploded, though the cause was unknown.

An estimated 4 to 6 million land mines and other munitions left over from three decades of armed conflict kill or maim scores of Cambodians each year, even though combat ended in the late 1990s.