A pickup truck carrying Afghan troops ran over a land mine Saturday, killing four soldiers and wounding five in southwestern Afghanistan, a government official said.

The truck apparently veered off a road into a field in a heavily mined area about 60 miles south of Chahar Burjak, the capital of Nimroz province, said Mohammad Sawaz Sudat, spokesman for the provincial government. The area is in the desert close to the Pakistani border.

The mine was Russian in origin and probably was laid during the 1979-1989 Soviet occupation of Afghanistan, Sudat said. Officials believed the explosion was not related to terrorist attacks by remnants of the former Taliban regime and Al Qaeda who are active in parts of southern Afghanistan, he said.

"This was an old mine so we've ruled out any link to the Taliban or Al Qaeda," Sudat said.

On Friday, a device, possibly a land mine, exploded beneath a passenger bus in Kandahar province, some 250 miles from the site of Saturday's blast. Local officials have detained seven people for questioning in that attack, which officials said likely was the work of Taliban fugitives or forces loyal to renegade warlord Gulbuddin Hekmatyar.

The soldiers were on their way to help with opium eradication work when the blast occurred at about 1:30 p.m., Sudat said.

Afghanistan has been attacking widespread poppy cultivation in hopes of losing its reputation as the world's prime source of opium and heroin.

Afghanistan is considered the most heavily mined country in the world, a legacy of two decades of near-continuous warfare, first against Soviet occupiers and then between rival Afghan factions struggling for supremacy.

About 200,000 Afghans have been killed or wounded by mines. New casualties are reported weekly and mine-clearing efforts are expected to take decades.