PESHAWAR, Pakistan, Dec 16 (Reuters) - Taliban gunmen in Pakistan took hundreds of students and teachers hostage on Tuesday in a school in the northwestern city of Peshawar, military officials said.

A Reuters journalist at the scene heard heavy gunfire from inside the school as soldiers surrounded it. Helicopters swooped overhead and a fleet of ambulances ferried wounded children to hospital.

The Lady Reading Hospital in Peshawar said the hospital had received the bodies of three students and was treating 28 injured students and two male teachers.

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

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.

A teacher said that the attackers targeted the school while exams were taking place.

"After half an hour of the attack, the army came and sealed the school," a teacher told a private television channel outside the besieged school.

"We were in the examination hall when the attack took place," he said. "Now the army men are clearing the classes one by one."

Taliban spokesman Muhammad Umar Khorasani told Reuters his group was responsible for the attack.

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

"It's a revenge attack for the army offensive in North Waziristan," he said, referring to an anti-Taliban military offensive that began in June.