The people of Peshawar are "very scared and very afraid," an officer for a Pakistani Christian advocacy group told Christian Today in the wake of today's school attack.

Speaking from Peshawar, about 190km West of Islamabad, the officer for the British Pakistani Christian Association (BPCA) confirmed that many children died in the violence, some of whom were Christians.

"Things are very bad for our city," he said, after a spending the day at the scene of the incident. "People are very scared...very afraid," he added, noting that the community is worried about possible future attacks.

Speaking from London, BPCA's chairman Wilson Chowdhry said today's attack, in which at least 126 people were killed, is a "devastating blow to the nation of Pakistan".

"The people of Peshawar must be terrified, so our heart goes out to all of them," he told Christian Today. "We intend to prvide aid and assistance where we can to help these people get their lives together again. It just smacks of the malevolence and hatred that the Taliban have, and I hope that any supporters they do still have, the last vestiges that remain, have relinquished any association with them.

"We're hoping, waiting and praying that the number of deaths does not increase," Chowdhry added.

Human rights activist and campaigner Malala Yousafzai has also condemned today's attack.

"I am heartbroken by this senseless and cold blooded act of terror in Peshawar that is unfolding before us," she said in a statement today.

"Innocent children in their school have no place in horror such as this. I condemn these atrocious and cowardly acts and stand united with the government and armed forces of Pakistan whose efforts so far to address this horrific event are commendable.

"I, along with millions of others around the world, mourn these children, my brothers and sisters – but we will never be defeated."

Malala was 14 when she was shot in the head by a Taliban gunman in 2012. She now campaigns extensively for female education, and in October became the youngest-ever Nobel Prize laureate.

As more information about today's attack becomes available, it is clear that that it is the bloodiest insurgent attack in Pakistan in years.

At least 126 people, most of them children, were killed when Taliban gunmen stormed a school in the Pakistani city of Peshawar, taking hundreds of students hostage. According to NBC News, a military source confirmed that a teacher was set on fire in front of students and the children were made to watch.

Witnesses have reported that many children were shot in the head or chest, and there have been "heavy" explosions.

Troops surrounded the building and an operation was underway to rescue children still trapped inside, the army said.

Hours into the siege, three explosions were heard inside the military-run high school, and a Reuters journalist at the scene said he heard heavy gunfire.

Outside, as helicopters rumbled overhead, police struggled to hold back distraught parents who were trying to break past a security cordon and get into the school.

Bahramand Khan, director of information for the regional Chief Minister's Secretariat, said at least 126 people were killed and 122 wounded.

"It may rise," he said, adding that more than 100 of the dead were school children. A local hospital said the dead and wounded it had seen were aged between 10 and 20 years old.

The hard-line Islamist Taliban movement immediately claimed responsibility.

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

It was not clear whether some or all of the children were killed by gunmen, suicide bombs or in the ensuing battle with Pakistani security forces trying to gain control of the building.

An unspecified number of children were still being held hostage in the school, a provincial official said, speaking some three hours after the siege began.

The Pakistani Taliban, who are fighting to topple the government and set up a strict Islamic state, have vowed to step up attacks in response to a major army operation against the insurgents in the tribal areas.

They have targeted security forces, checkpoints, military bases and airports, but attacks on civilian targets with no logistical significance are relatively rare.

In September, 2013, dozens of people, including many children, were killed in an attack on a church, also in Peshawar, a sprawling and violent city near the Afghan border.

With the rescue operation under way, the situation remained fluid, with contradictory reports about what was happening inside the school and witness accounts difficult to come by.

"An army doctor was visiting us teaching us about first aid when attackers came from behind our school and started firing," one student told Pakistan's Dunya Television.

"Our teachers locked the door and we ducked on the floor, but they [the militants] broke down the door. Initially they fired in the air and later started killing the students, but left the hall suddenly.

"The attackers had long beards, wore shalwar kameez [traditional baggy clothes] and spoke Arabic."

The army said five Taliban militants had been killed and that they were searching for any remaining gunmen. The Taliban had earlier said they had sent six insurgents with suicide vests to attack the school.

Prime Minister Nawaz Sharif condemned the attack and said he was on his way to Peshawar.

"I can't stay back in Islamabad. This is a national tragedy unleashed by savages. These were my kids," he said in a statement.

"This is my loss. This is the nation's loss. I am leaving for Peshawar now and I will supervise this operation myself."

Military officials at the scene said at least six armed men had entered the military-run Army Public School. About 500 students and teachers were believed to be inside.

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

(Additional reporting by Reuters)