A US drone strike in a troubled Pakistani tribal region near the Afghan border on Sunday missed its target -- a militant vehicle-- allowing rebels to flee, local security officials said.

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 Islamist 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," a senior security official told AFP.

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, according to an AFP tally.

Pakistan tacitly cooperates with the bombing campaign, which US officials say has severely weakened Al-Qaeda's leadership and killed a number of high-value targets, including the former Pakistani Taliban chief Baitullah Mehsud.

The United States has been increasing pressure on Pakistan to crack down on Islamist havens along the Afghan border, a hub of Taliban and Al-Qaeda linked militants fighting across the border in Afghanistan.

Pakistani commanders have not ruled out an offensive in North Waziristan, but argue that gains in South Waziristan and the northwestern district of Swat need to be consolidated to prevent their troops from being stretched too thin.