US drones fired missiles into a compound and a motorbike in Pakistan's northwestern tribal belt, killing at least six militants on Thursday, local security officials said.

It was the fifth American drone strike reported in Pakistan in a week, the heaviest concentration since last August, according to an AFP tally.

Pakistani security officials said US drones fired four missiles -- two at a house and another two at a nearby motorbike, killing four militants in the house and two on the bike.

The attack took place in Heso Khel village on the edge of Mir Ali town, around 35 kilometres (21 miles) east of Miranshah, the main town in North Waziristan, a notorious stronghold of Taliban and Al-Qaeda-linked militants.

"Six drones were in the sky at the time of the attack. One drone fired two missiles at a house while a second drone fired another two at a motorcycle," a security official based in Miranshah told AFP.

"The compound was completely destroyed, bodies of all those killed were badly mutilated," the official added, putting the death toll at six.

A local resident said militants were seen collecting burnt bodies.

A security official in the northwestern city of Peshawar confirmed the attack but said the the identity of the dead was not yet known.

Thursday's strike brings to 36 the number of militants reported killed in Pakistan in five drone strikes since January 3.

The covert strikes are publicly criticised by the Pakistani government as a violation of sovereignty but American officials believe they are a vital weapon in the war against Islamist militants.

Few of the victims are publicly identified, but on January 3 Pakistani officials said warlord Mullah Nazir was killed in a strike on South Waziristan.

He sent fighters into battle in Afghanistan and was accused of sheltering Al-Qaeda in South Waziristan.

According to the London-based Bureau of Investigative Journalism, between 2,627 and 3,457 people have been reportedly killed by US drones in Pakistan since 2004, including between 475 and nearly 900 civilians.