Militants have targeted a girls' school near the Pakistani city of Peshawar which has been prey to reprisal attacks by pro-Taliban militia.

The school was heavily damaged in the Thursday attack in the Budaber area, about 10 kilometers (six miles) south of Peshawar, AFP quoted local police chief Abdul Ghafoor Afridi as saying.

Four classrooms were completely destroyed and three others were damaged in the attack which left no casualties as schools were closed near summer break.

Afridi said the explosives used to blow up the school amounted to at least 40 kilograms (88 pounds) while another police official said militants had used a timed detonator device.

The incident is the latest in the round of an intimidation campaign launched by militants who have threatened Islamabad with bombings and violence in a bid to force the government into halting military operations in Swat valley.

The attack has raised serious concerns in Pakistan where the government has since late April used schools to shelter some of the 2.4 million people displaced by the conflict.

Last Thursday, four blasts -- two simultaneous explosions in Qisa Khwani and Kabari Bazar in Peshawar followed by two separate ones outside the capital of the North Western Frontier Province city -- left at least fourteen people killed and more than one hundred others injured.

The incidents came one day after militants killed almost thirty people and injured 300 in an attack that flattened a police building and ripped through an intelligence office in the eastern city of Lahore.

Militants have destroyed 191 schools, including 122 schools for girls, in the district of Swat where blacklisted Maulana Fazlullah has been leading extremists for the past two years.

Pakistan's Interior Ministry has offered a bounty of PKR 50 million (USD 622,500 dollars) for Fazlullah, dead or alive, and PKR 10 (USD 124,500) million rupees for each of his 15 aides.

Pro-Taliban militias on Thursday released all 50 remaining staff and students who had been kidnapped in the North Waziristan tribal area three days before.

The kidnapping happened on Monday when nearly 400 students in a convoy of 30 minibuses were held up at gunpoint en rout to the town of Bannu. Sources said most of the abductees had been rescued by the government troops on Tuesday.

MRS/HGH