The military meanwhile reported that a roadside bomb attack killed two NATO soldiers in southern Afghanistan, where around 70 militants were killed in various incidents in the past 24 hours as insurgency-linked violence picks up.

The truck, loaded with firewood, overturned overnight about 30 kilometres (19 miles) south of the capital in Logar province and exploded as authorities were trying to remove the vehicle in the early morning, they said.

The attack comes as Western militaries boost their troop deployments to Afghanistan ahead of key presidential and provincial council elections scheduled for August 20.

"In the explosion today 21 civilians and four policemen have been martyred," provincial police chief Ghulam Mustafa Mohsini told AFP.

The students were struck as they were going to schools in the area.

Nothing remained of the truck and three shops nearby were reduced to rubble, an AFP reporter at the site witnessed. Police said some bodies were burned beyond recognition.

The education ministry said it was working to confirm how many students were killed but believed at least 13 had died. Ministry spokesman Asif Nang, who was at the scene, could not immediately give details.

The interior ministry in Kabul also said 25 people were dead.

"Police went in the morning to see and open the road. As police arrived at the scene the truck exploded and unfortunately killed 21 civilians and four policemen," ministry spokesman Zemarai Bashary told AFP.

It was not clear if the truck was headed to the capital to carry out an attack or had been intended to explode where it did, the official said.

The blast took place on a main road from southern and eastern Afghanistan that heads into Kabul.

The vehicle, heading towards the capital, appeared to have been deliberately overturned, said provincial government spokesman Din Mohammad Darwish.

"It seems that the explosives were remotely detonated as a crowd gathered around the truck in the morning," he said.

There was no immediate claim of responsibility.

Afghanistan has for years suffered a wave of bombings by Islamist insurgents, including those waged by the Taliban, who were in government between 1996 and 2001 until they were ousted by a US-led invasion.

The rising violence prompted US President Barack Obama to pledge 21,000 more troops, most of whom are already in the country as part of a sweeping new strategy to try to stabilise the country ahead of the elections next month.

Nearly 4000 newly deployed US Marines last week launched a major operation in Taliban strongholds in the southern province of Helmand where thousands of British troops are also operating.

Two international soldiers were killed by an improvised explosive device in the south on Wednesday, said the NATO-led force under which the Marines are serving. It gave no details.

In a separate incident Thursday, militants attacked a district headquarters in the southern province of Zabul, sparking a clash in which 15 Taliban were killed, provincial police chief Abdul Rehman Sarjung told AFP.

In the same province, 30 insurgents who had been planting bombs in a road were killed in an Afghan army ambush, said the army commander for southern Afghanistan, General Shair Mohammad Zazai.

AFP

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