According to a statement released on Thursday by officials, Islamist militants killed at least 21 people in what may have been a suicide attack on an army convoy in northwest Pakistan, where a Taliban-style movement has taken root.

The blast in the Swat valley set fire to a truck laden with ammunition a day after the military sent around 2,000 troops to the district in response to growing militant activity.

The bodies of the victims were badly burnt. Naveed said at least 17 of the dead were soldiers.

Gul Haleem, in charge of the casualty department at a hospital in the town of Saidu Sharif, said he had counted 21 corpses.

President Pervez Musharraf condemned the "dastardly terrorist attack" on the convoy, the state-run Associated Press of Pakistan reported.

State television reported that a firefight had broken out after the blast, the latest in a series of attacks on security forces in recent months which have killed hundreds of people. Witnesses said, though, it was the sound of ammunition exploding.

"When we reached near the truck it was burning. Flames were rising high into the sky. Ammunition was exploding. Police stopped us going near the truck," local resident Saeed Khan said.