Pakistani teenager Malala Yousafzai, joint winner of this year's Nobel peace prize for education campaign work and survivor of a Taliban attack in 2012, said she was devastated by a Taliban killing spree at a school in Peshawar that left over 130 students dead.

"My family and I are heartbroken after hearing the news that more than 100 innocent children and teachers have lost their lives in this recent attack on a school in Peshawar and we stand with all those families and all those children who are injured right now and who are suffering through this big trauma," Malala, who now lives in central England, said in a video statement.

Nine staff members were also killed after Taliban gunmen broke into the school and opened fire, witnesses said, in the bloodiest massacre the country has seen for years.

More than eight hours after militants slipped into the heavily guarded compound through a back entrance, the army declared the operation to flush them out over, and said that all nine insurgents had been killed.

The attack on a military-run high school attended by more than 1,100 people, many of them children of army personnel, struck at the heart of Pakistan's military establishment, an assault certain to enrage the country's powerful army.

Wounded children taken to nearby hospitals told Reuters most victims died when gunmen, suicide vests strapped to their bodies, entered the compound and opened fire indiscriminately on boys, girls and their teachers.

"Now it is the time that we unite and I call upon the international community, leaders in Pakistan, all political parties and everyone that we should stand up together and fight against terrorism and we should make sure that every child gets safe and quality education," the teenage activist said.

The Pakistani Taliban have vowed to step up attacks in response to a major army operation against the insurgents in the tribal areas.