PESHAWAR - Six militants were killed and two others injured when unmanned US predator drones fired six missiles on a mud house in North Waziristan Agency on Thursday.

The attack was carried out in Datta Khel area of North Waziristan Agency in the early hours of Thursday. The men killed are believed to be militants of Hafiz Gul Bahadar group, however, there was no immediate government response that who were behind the drone strike.

It was the second strike launched during the military operation Zarb-e-Azb while fourth since the US resumed its campaign following a six-month gap after the PTI workers halted Nato supply routes in Peshawar.

Thursday's strike in the Datta Khel district also injured two militants, sources said and added that site of the strike was about 28 miles west of the regional capital, Miranshah, the main town in the lawless North Waziristan region near the Afghan border.

Agencies, however, reported that Missiles from two US drones slammed into a mud house and killed six suspected militants, four of them foreign fighters and two local cadres who had fled Miranshah before the launch of the offensive.

Two US drones fired four missiles into a compound in Doga Macha Madda Khel, a village close to Datta Khel town, local security officials said.

"The compound and a vehicle parked inside were completely destroyed and killed at least six militants," the official told AFP.

Doga Macha Madda lies in difficult, mountainous terrain which the official said the militants had hoped to use as cover.

Another security official in Peshawar confirmed the attack and the casualties.

It is the third round of drone strikes to hit North Waziristan since the US resumed the campaign following a six-month hiatus. Washington reportedly suspended its drone programme in December to give Islamabad time to pursue a dialogue process with the Tehreek-e-Taliban Pakistan (TTP) aimed at ending a seven-year insurgency that has claimed thousands of lives.

But a spectacular, bloody attack on Karachi airport on June 9 that left dozens dead sank the peace efforts and prompted the army offensive.

US drone strikes resumed a few days after the Karachi attack, though Pakistani officials insist they have not given their approval.

A senior officer took reporters on a tour of the region on Wednesday to underscore what the military says is a successful offensive to bring under control 80 percent of Miranshah, North Waziristan's main town.

"It's about 80 percent clear from the terrorists but it's still not clear of IEDs (improvised explosive devices) - it will take more time for combat engineers to clear the IEDs," General Zafarullah Khan told AFP in Miranshah.

Major General Asim Bajwa, the head of the military's media wing, said that before the onslaught the town had become a virtual supermarket for fighters wanting to equip themselves with guns and explosives.

"This looked like a terrorist economy, a whole chain of things being sold in the city," Bajwa told AFP.

"You could find a shop selling IEDs, another shop selling fuses, and a third selling cords and detonators."

Soldiers showed journalists what they described as a "suicide bomber training centre" complete with explosive vests lined up against the walls.

In all, the army said they had recovered 23 tonnes of explosive material in Miranshah, including IEDs, mines and rocket-propelled grenades.