Some 384 prisoners including militants escaped early Sunday from a jail in northwestern Pakistan after an attack by insurgents armed with guns, grenades and rockets, officials said.

More than 150 heavily-armed Islamist militants stormed the central prison outside the restive northwestern town of Bannu bordering the lawless tribal regions where Taliban and Al-Qaeda linked militants are known to operate.

Ehsanullah Ehsan, spokesman for the Tehreek-e-Taliban Pakistan (TTP) militant group claimed responsibility for the attack. "We attacked the Bannu prison and got our special members freed," Ehsan told AFP.

"In a couple of days when all of them have reached their designated places we will issue details about them. At the moment I cannot give you exact numbers," he said.

The attack started at around 1:00 am (2000 GMT Saturday) and continued for two hours, with militants in cars and pick-up trucks shooting and lobbing grenades to force their way into the prison, a security official told AFP.

"Some 384 prisoners, including some hardcore militants, have escaped during the attack," the official said, asking not to be named.

"Preliminary information suggests that there were some 944 prisoners in the jail according to the tally late Saturday," he said.

Information minister for Khyber Pakhtunkhwa province, Mian Iftikhar Hussain, told AFP there were "at least 20 dangerous prisoners and some militants", among the escapees.

"We will investigate why militants were able to carry out such an attack successfully and what the security was doing," he said.

A large number of militants had recently been moved to the jail from neighbouring Kohat and Lakki Marwat prisons, which are being converted into centres to rehabilitate former insurgents, the security official said.

A former member of the airforce sentenced to death for an attack on former president Pervez Musharraf was among the escaped militants, he said.

Police confirmed the attack but did not give any figures for the escapees, saying they were still investigating.

Senior Bannu police official Iftikhar Khan told AFP that at least three police officials were wounded in the attack.

The attackers outnumbered the security forces at the prison and militants fled before reinforcements reached the jail, he said.

Shafiq Ahmed, another police official said that security forces had imposed a cordon and a search operation was launched in the area.

"We have arrested four of the escaped prisoners," Ahmed told AFP.

Pakistan's tribal districts near the Afghan border are rife with homegrown insurgents, Al-Qaeda operatives and Taliban, who are understood to use rear bases in Pakistan to plot attacks in neighbouring Afghanistan.

The mountainous region lies outside direct government control and US drone strikes on militant commanders in the region are a key plank in the US strategy to defeat Al-Qaeda and reverse the insurgency in Afghanistan.

Islamist militants have killed more than 4,900 people across Pakistan since government troops raided an extremist mosque in Islamabad in July 2007. (AFP)