Militants disguised as police and armed with bombs and guns attacked a prison in northwest Pakistan holding 40 "high profile" inmates Monday night in an apparent attempt to free their colleagues, officials said.

The attack in the town of Dera Ismail Khan began around midnight with a huge explosion, said intelligence officials, speaking on condition of anonymity because they were not authorised to talk to reporters. The militants then detonated a series of smaller bombs to destroy the prison's boundary wall.

At least eight attackers wearing police uniforms stormed inside the prison once the walls fell, said the officials. Security forces engaged the attackers, who were chanting "God is great" and "Long live the Taliban."

"We are trying to bring the situation under control," said Dera Ismail Khan police chief Sohail Khalid. "The attack is still continuing."

Khalid Abbas, head of the prison department in surrounding Khyber Pakhtunkhwa province, said the jail held 40 "high profile" prisoners.

"We are not sure if any of them escaped," Abbas said.

Authorities received a letter threatening an attack on the prison, but they didn't expect it so soon, said Abbas. Army troops encircled the prison and exchanged fire with the attackers, he said, but security forces were having trouble distinguishing the militants in the dark because of their police uniforms.

A resident, Sharafat Khan, said the initial blast was so loud that "it rattled every house in the neighborhood."

"I'm still hearing gunshots," Khan said.

One prison official, Gul Mohammad, said he had just walked out of the prison at the end of his shift when two militants armed with AK-47s shot him. There are other officials who have been wounded, he said from a hospital bed, although the casualty toll was unclear.

No one has claimed responsibility for the attack, but suspicion will likely fall on the Pakistani Taliban and their allies. The Taliban have been waging a deadly insurgency against the government for years that has killed thousands of security personnel and civilians.

Dera Ismail Khan is located on the edge of Pakistan's semiautonomous tribal area, the main sanctuary for Taliban and al-Qaida militants.

In April 2012, Taliban militants armed with automatic weapons and rocket-propelled grenades battled their way into a prison in the city of Bannu in northwest Pakistan, freeing close to 400 prisoners, including at least 20 described by police as "very dangerous" insurgents.

One of the militants freed in that attack, Adnan Rasheed, recently gained attention by writing a letter to teenage education activist Malala Yousafzai, who was shot in the head by the Taliban last year in an attempt to kill her. Rasheed said he wished the attack hadn't happened, but told Malala that she was targeted for speaking ill of the Taliban.