(Updates with cause of blast)
MINGORA, Pakistan, Oct 25 (Reuters) - A suspected suicide bomber killed 21 Pakistanis in an attack on an army convoy on Thursday in the northwest, where a Taliban-style movement has taken root, officials said.
The bombing 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.
"It appears to be a suicide attack because there is no crater at the site of the blast," Malik Naveed, commander of the paramilitary Frontier Corps in Swat, told Reuters by telephone.
A senior government official said investigators were trying to determine whether it was a car bomb or the attacker was on foot and blew himself near the truck.
"There is a badly burnt car that hit the truck but we are not sure whether it belonged to a passerby and hit the truck accidentally or the attacker was sitting in it," the official said on condition of anonymity.
The bodies of the victims were badly burnt. Naveed said at least 17 of the dead were his soldiers.
Gul Haleem, in charge of the casualty department at a hospital in Saidu Sharif, said he had counted 21 corpses.
U.S. ally President Pervez Musharraf condemned the attack, the state-run Associated Press of Pakistan reported.
"IT WAS BURNING"
Witnesses said the truck caught fire after the bombing.
"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," resident Saeed Khan said.
Swat has seen a surge in militant activities since Maulana Fazlullah, a pro-Taliban cleric, reportedly launched an illegal FM radio calling people to jihad or Muslim holy war.
Fazlullah is de facto head of a pro-Taliban group, Tehrik-e-Nifaz-e-Shariat-e-Mohammadi (TNSM) or Movement for the Implementation of Mohammad's Sharia Law, which was banned by Musharraf in January 2002.
Militants have attacked security forces and carried out bomb attacks in recent months in the scenic valley in North West Frontier Province, and have been forcing residents there to follow a strict Islamic code.
Pakistani tribal areas have been a hotbed of support for al Qaeda and Taliban militants who have fled Afghanistan. Thousands of soldiers and militants have been killed in battles in these regions.
Violence has escalated across Pakistan since July, when militants scrapped a peace deal and the army raided a radical mosque in the capital, Islamabad. (Additional reporting by Zeeshan Haider and Augustine Anthony)
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