Taliban gunmen have killed 126 people, including at least 100 children, in a school attack in Peshawar, Pakistan.

Six men, some wearing suicide vests, stormed the army-run school, according to military officials, and soldiers surrounded the building.

Reports say all six of the militants have now been killed.

Around 500 children and teachers were believed to be inside, with many students taking exams at the time. At least 122 people are thought to have been injured.

An undisclosed number of teachers and pupils were held hostage by the Tehreek-e-Taliban militants before they were all killed by the army.

"Sixth terrorist killed in last block," Major General Asim Bajwa tweeted. "IEDs (bombs) planted by terrorists hamper speed of clearance."

Police were struggling to hold back distraught parents trying to break past a cordon and get to the school when three loud explosions went off, police officials said.

"My son was in uniform in the morning. He is in a casket now," wailed one parent, Tahir Ali, as he came to the hospital to collect the body of his 14-year-old son Abdullah. "My son was my dream. My dream has been killed."

Education campaigner and Nobel peace prize winner Malala Yousafzai, who was shot by the Taliban, said: "I am heartbroken by this senseless and cold-blooded act of terror in Peshawar that is unfolding before us."

Reporter Neville Lazarus is in Delhi and has more details on the attackers.

"All the children were bleeding"

Video and photos showed other young children in their green uniforms being led away from the school by soldiers and an army helicopter flying overhead.

"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 our pain."

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

The school is located on the edge of a military cantonment in Peshawar, but the majority of the students are civilian.

Pakistan's prime minister Nawaz Sharif, who has arrived in the area, called the massacre a "national tragedy".

One of the wounded students, Abdullah Jamal, said that he was with a group of 8th, 9th and 10th graders who were getting first-aid instructions and training with a team of Pakistani army medics when the violence began for real.

When the shooting started, Abdullah, who was shot in the leg, said nobody knew what was going on in the first few seconds.

"Then I saw children falling down who were crying and screaming. I also fell down. I learned later that I have got a bullet," he said, speaking from his hospital bed.

"All the children had bullet wounds. All the children were bleeding," Abdullah added.

A teaching assistant there has been describing what happened.

A Taliban spokesman told the Reuters news agency the attack was "revenge" for an army offensive against the group in North Waziristan.

"Our suicide bombers have entered the school, they have instructions not to harm the children, but to target the army personnel," he said.

These students have been describing what happened.