Pakistani army troops backed by helicopter gunships on Thursday kept up their offensive against armed followers of a pro-Taliban cleric in Swat valley, killing at least 20 militants across the restive region.

Troops supported by artillery and mortar fire launched an operation to regain control of Alpuri, the headquarters of Shangla district, which had been taken over by the militants led by Maulana Fazlullah earlier this week.

At least 12 militants were killed in this action and more of them were seen fleeing from Alpuri. The operation was also aimed at clearing the Besham-Shangla road, military spokesman Maj Gen Waheed Arshad said.

Helicopters gunships also targeted militant positions north of Kabal and Kuza Banda, killing eight militants and destroying an ammunition dump and mortar positions there, Arshad said.

The figure for casualties among the militants was based on information from local sources and intercepts of militant communications.

The claim by militants that they had killed some soldiers and captured Frontier Constabulary troops was "totally baseless and false", Arshad said.

On Wedmesday, 33 militants and two soldiers were killed in clashes in the troubled north-western valley.

Over 200 people, including 150 militants, have died in fierce fighting in Swat since the federal government rushed thousands of troops to the area last month to quell the activities of Fazlullah and his men.