A soldier escorts children away from the Army Public School in Peshawar Khuram Parvez/Reuters

At least 100 people, 80 of them children, have been killed in a Taliban assault on an army-run school in Peshawar, Pakistani officials say.

Six Taliban militants opened fire at The Army Public School and earlier were holding 500 students and teachers hostage - the majority of which have now been evacuated.

But it is still not known how many are being held inside the building.

"We want them to feel the pain"
- Taliban spokesman

One member of the Pakistani paramilitary forces was also killed in the raid, which started in the early hours of this morning, according to Jamil Shah, a spokesman for Lady Reading Hospital in Peshawar.

The Pakistani Taliban said it was a "revenge" attack following an army offensive against Islamic extremists in North Waziristan and in nearby Khyber.

"We selected the army's school for the attack because the government is targeting our families and females," said Taliban spokesman Muhammad Umar Khorasani, according to Reuters. "We want them to feel the pain."

Pervaiz Khattak, chief minister of Khyber Pakhtunkhwa, said 84 children had been killed.

"In CMH (Combined Military Hospital) there are around 60 and there are 24 dead in Lady Reading (hospital)," he told local media.

Mudassir Awan, a worker at the school, said he saw six people scaling the walls of the school.

"We thought it must be the children playing some game," he told Reuters. "But then we saw a lot of firearms with them.

"As soon as the firing started, we ran to our classrooms," he said. "They were entering every class and they were killing the children."

More than 25 ambulances have been called in to help the rescue operation, which is still in progress inside the school, according to local media reports.

Two male teachers are among the 28 casualties being treated in hospital.

"Many are in the operation theatre now in critical condition, undergoing treatment," said hospital official Ejaz Khan.

Gunmen stormed the school while an exam was taking place, said a teacher.

"We were standing outside the school and firing suddenly started and there was chaos everywhere and the screams of children and teachers," said a school bus driver.

Prime Minister David Cameron expressed his horror at the unfolding tragedy.