The missile strike took place in Azam Warsak town, 20 kilometres (12 miles) west of Wana, the main town of the South Waziristan tribal district, where the Pakistani military launched an operation against militants in 2009.

"US drones first fired two missiles targeting a militant vehicle but they failed to hit, allowing rebels, who were said to be over four in number, to run away," according to a senior security official.

He said that two more missiles fired from a drone hit the vehicle but failed to destroy it.

Another security official in the area confirmed the strike, adding that at least four of the unmanned aircraft had been flying in the area on Sunday morning.

He said that identity of militants on board the vehicle was unknown.

The United States does not confirm drone attacks, but its military and its Central Intelligence Agency (CIA) operating in Afghanistan are the only forces that deploy them in the region.

The covert US strikes cause anti-American hostility among the Pakistani public, who see foreign military action on Pakistani soil as a violation of national sovereignty.

Missile attacks doubled in the tribal areas last year as the campaign was stepped up, with more than 100 drone strikes killing over 670 people in 2010 compared with 45 strikes that killed 420 in 2009.