Reports say more than a dozen other troops and several civilians were injured in the blasts. Taleban sources ruled out sabotage as a possible cause.

The blasts happened at a military base on the road out of Mazar-e-Sharif towards Shibergan. Taleban forces captured Mazar-e-Sharif in August last year.

Taleban truck: The explosion happened when loading arms onto a truck

The Pakistan-based Afghan Islamic Press said the first explosion occurred as ammunition was being loaded onto a truck.

The blast caused a fire which took two hours to control, it said, adding that a series of smaller explosions also occurred at the site.

During the long civil war in Afghanistan many ammunition dumps have exploded, including several in Kabul and one very large one in the eastern city of Jalalabad in 1997.

Often, the causes of these explosions have remained unknown.

Fresh fighting

Meanwhile, the opposition Northern Alliance said it had launched an attack in north Afghanistan and advanced 15km into Taleban-held territory.

A spokesman for the group, Mohammad Habeel, said the operation was launched at Pul-e-Khumri in Baghlan province with forces loyal to opposition commander Ahmad Shah Masood seizing the high ground around the Kelagai desert.

"From there, heavy machine gun fire can threaten the main highway linking Kabul with the northern capital Mazar-e-Sharif," he said.

Commander Masood launched a counter-offensive against the ruling Taleban on Friday aimed at retaking ground lost to the militia since it launched its northern campaign two weeks ago.