Thirty-five militants were killed as security forces launched a fresh offensive against armed followers of a pro-Taliban cleric in the restive Swat valley of north-western Pakistan, where hundreds of people fled their homes following instructions from the army.

Military spokesman Maj Gen Waheed Arshad said the security forces had inflicted "heavy casualties" on militants in Shangla district adjoining Swat during operations on Monday night and at least 20 militants had been killed.

Another 15 militants were killed in Swat, where the army's operations are continuing, he told Dawn News channel.

There were no reports of casualties among the security forces and Arshad dismissed militant spokesman Sirajuddin's claim that several soldiers had been killed.

As helicopter gunships and artillery pounded militant positions in mountains around several towns and villages in the Swat valley, security forces advised people to leave their homes to avoid casualties.

This triggered a fresh exodus of hundreds of people from certain villages in the area, local residents said.

Interior ministry spokesman Javed Iqbal Cheema told reporters that people in certain areas were advised to leave to "avoid collateral damage as militants were forcibly taking shelter in their houses and firing on the security forces".

Arshad said the army had launched relief operations for the displaced people.

The army on Tuesday continued advancing from Rheem Sar Bande towards Alpuri, the headquarters of Shangla district which was taken over by the militants last week.

The army is now in control of the strategic Besham-Alpuri road.

Twelve militants were killed and 40 injured in clashes with the advancing troops earlier.

Among the dead was a top militant named Muhammad Amin who was involved in a bank robbery, said a statement from the army.

Artillery guns also pounded militant positions around Kabal, a stronghold of the armed followers of radical cleric Maulana Fazlullah.

The shelling set forests on hills on fire and TV channels aired footage showing columns of smoke billowing into the skies.

The army said intercepts of wireless communications between militants on Tuesday night had revealed that they had suffered "heavy losses and are now in a state of despair".

The military is also engaging local elders and 'nazims' to sway public opinion in favour of the ongoing operations in Swat and adjoining areas.

Close to 300 people, a majority of them militants, have been killed in fighting between Maulana Fazlullah's men and security forces since late October, when the federal government rushed thousands of troops to quell the activities of the radical cleric.

After paramilitary forces and police were unsuccessful in tackling the militants, President Pervez Musharraf [Images] last week ordered the army to take a lead role in the operations.

The army has amassed some 15,000 troops for a fresh offensive against the militants and commanders are hopeful of restoring order in Swat by December.