A fragile truce in the Swat valley of northwest Pakistan collapsed on Wednesday as troops targeted militant hideouts with artillery and helicopter gunships, killing at least 20 insurgents.

The attacks on armed followers of pro-Taliban cleric Maulana Fazlullah in and around Matta were launched after the militants carried out overnight attacks on police posts near Kabal and Saidu Sharif, officials said.

Twenty militants were killed and six others injured in the ambush, officials said.

There were reports that the militants, who were armed with rockets and assault rifles, fired at a helicopter flying over Matta town, prompting the security forces to target their positions with mortars. TV channels reported that there were several explosions and exchanges of fire in the mountains around Matta.

The militants had called a unilateral truce on Monday to allow both sides to retrieve and bury bodies of those killed in clashes since October 26. Officials, however, believe that the militants had used the lull to regroup and strengthen their positions.

Heavily armed militants were patrolling the streets in several towns and villages. There were also reports that the militants had forcibly taken away several cars from automobile showrooms as well as from police posts in the region.

Officials said the militants had captured the civil hospital in Manglore and several other hospitals. They also fired two rockets at a police station in Kabal and several rockets at posts occupied by security forces in the Frontier House.

Thousands of troops were deployed last week in Swat valley, till recently a popular tourist destination, to counter the activities of Fazlullah and his men. The militants retaliated on October 25, when a suicide bomber blew up a truck carrying security personnel, killing 22 people.

The following day, troops backed by helicopter gunships attacked Fazlullah's madrassa, sparking three days of fighting that left at least 60 militants and 20 security personnel and civilians dead.

Fazlullah, known as "FM Maulana" for his broadcasts calling for jehad from an illegal radio station, and his men are running a parallel government in nearly 60 villages in the Swat region.