Dozens of militants attacked a Pakistani paramilitary checkpoint overnight, killing two soldiers and sparking clashes which left up to 21 of the militants dead, officials said Friday.

Security officials claimed the militants were killed when troops retaliated with artillery and heavy weapons, but no independent confirmation of the death toll was possible in an area cut off to journalists and aid workers.

The attack took place in a Taliban hotspot of the tribal badlands when a group of more than 50 militants attacked a checkpost in the Khadizai area, around 75 kilometres (47 miles) southwest of Kalaya, which is the main town of Orakzai tribal district.

"The attack came at around 1:00 am. The militants killed two soldiers and injured 11. We retaliated and killed 21 militants," a senior security official told AFP on condition of anonymity.

"The clash continued for around two hours," he added.

A local administration official said the injured soldiers and dead bodies were later flown by helicopter to a military hospital.

Khadizai is in Upper Orakzai, most of which lies in Taliban hands and is the scene of frequent clashes between security forces and Islamist militants.

Pakistan's seven tribal districts near the Afghan border are rife with homegrown insurgents and are strongholds of Taliban and Al-Qaeda operatives.

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