DERA ISMAIL KHAN, Pakistan â€"  Taliban militants armed with automatic weapons and rocket-propelled grenades battled their way into a prison in northwest Pakistan on Sunday, freeing close to 400 prisoners, including at least 20 described by police as "very dangerous" insurgents, authorities and the militants said.

The raid by more than 100 fighters was a dramatic display of the strength of the insurgency gripping the nuclear-armed country. The escaped prisoners may now rejoin the fight, giving momentum and a propaganda boost to a movement that has killed thousands of Pakistani officials and ordinary citizens since 2007.

The attackers stormed the prison before dawn in the city of Bannu close to the Afghan border, said police officer Shafique Khan. They used explosives and hand grenades to knock down the main gates and two walls, said Bannu prison superintendent Zahid Khan.

"They were carrying modern and heavy weapons," said Zahid Khan. "They fired rockets."

Once inside the building, the attackers headed straight to the area of the prison where death-row prisoners were being kept, he said. They fought with guards for around two hours, setting part of the prison on fire before freeing the 380 inmates, including at least 20 "very dangerous Taliban militants," said Shafique.

One escaped prisoner, Adnan Rashid, was on death row for his involvement in an assassination attempt against former Pakistani President Pervez Musharraf, said Zahid Khan.

The prison in Bannu housed 944 inmates.

A Taliban spokesman, Asimullah Mehsud, claimed the movement's fighters freed 1,200 of their comrades. The group is known to make exaggerated claims.

Pakistan's military has launched a series of operations against the Pakistani Taliban group in the northwest, where it is strongest and has forged alliances with al-Qaida and other transnational militant movements based there along the Afghan border.

The movement is closely linked to the Afghan Taliban, which is battling U.S. and NATO troops in Afghanistan.

Soldiers and police have killed or arrested hundreds of militants, but the insurgency has proved resilient. Insurgents have carried out suicide bombings and other attacks across the country in retaliation, raising doubts in some quarters over whether the county can survive. Prison breakouts like the one Sunday have been rare.

Bannu city is the main gateway to North Waziristan, the most militant-infested region along the border.