A high school for girls in Pakistan's north-west Swat district was destroyed after militants set it ablaze in an overnight raid, police and officials said on Sunday.

Around 50 rebels entered the school in the conflict-hit Charbagh area, located some 150 km from the city of Peshawar and used petrol bombs to destroy the library and nine other rooms in the building, the police said.

"The masked men directed me to remove all the copies of Koran (the holy book of Islamic religion) and then set the school on fire," said watchman Toti Gul.

The militants also planted homemade bombs in the school's science laboratory; however, they did not go off and were later defused by a bomb disposal squad, the watchman said.

A number of attacks on girls' schools have been reported in the past and these were believed to be carried out by pro-Taliban fighters commanded by a local radical cleric, Maulana Fazlullah.

Pakistani military launched a massive operation against the pro-Taliban militants in the picturesque mountain district late last year to flush Fazlullah's supporters out of several key towns and villages.

However, now there is a lull in direct clashes between the rebels and the military as the government has launched peace talks with the insurgents.

"The militants were bold enough to stay at the school for at least 20 minutes to see the building burn out," Gul said.

The attack came at a time when the annual examinations in the whole of Pakistan's North-West Frontier Province (NWFP) are in progress, and it is feared that hundreds of girl students would not be able to sit them, as the school was the only one in Charbagh.

Fazlullah used to broadcast radio transmissions through his illegal FM stations based in Imam Dheri village, discouraging the local population from educating girls and getting children immunised against polio, saying the UN-sponsored vaccination drive aimed at causing sexual impotence.

Authorities in Swat said though the government had reached a peace agreement with the militants in the valley that was once a haven for tourists, violence was again on the rise, particularly after the release of a firebrand cleric detained in Peshwar.

Pakistan saw a surge in militancy and violence, especially against its armed forces, after government troops cracked down on militants holed up in the Red Mosque in capital Islamabad. More than 110 people were killed in the commando raid.