A US drone Friday killed at least three militants when it fired missiles on a vehicle in a restive Pakistan tribal area bordering Afghanistan, security officials said.

The strike took place at Baghar village of South Waziristan tribal district, two security officials said.

"A US drone fired two missiles at a vehicle and at least three militants were killed," a senior security official told AFP.

Another security official and an intelligence official based in the region confirmed the attack and death toll.

The identities or affiliations of the militants were not immediately clear.

Although the United States does not publicly confirm drone attacks, its military and the CIA in Afghanistan are the only forces that deploy the unmanned Predator aircraft in the region.

More than 30 drone strikes have been reported in Pakistan since elite US forces killed Al-Qaeda chief Osama bin Laden in a suburban home near Pakistan's main military academy in Abbottabad, close to the capital, on May 2.

The raid humiliated Pakistan and prompted allegations of incompetence and complicity in sheltering bin Laden.

Pakistan is seen as a key ally for the United States in its fight against Islamist militancy, but relations soured after the bin Laden raid, which both countries say was carried out without Islamabad being warned.

Drone attacks are unpopular among many Pakistanis, who oppose the alliance with Washington and who are sensitive to perceived violations of sovereignty.

US officials have accused Pakistani intelligence of playing a double game with extremists, including the Afghan Taliban and the Haqqani network, in order to exert influence in Afghanistan and offset the might of arch-rival India.

Washington's pressure on Islamabad to launch a decisive military campaign in North Waziristan, as Pakistan has conducted elsewhere in the tribal belt, has so far fallen on deaf ears.

Pakistan on Thursday closed ranks against increasing US pressure for action against the Al-Qaeda-linked Haqqani network, refusing to be pressured into doing more in the war on terror.

Prime Minister Yousuf Raza Gilani brought together opposition leaders, members of his fragile coalition and military commanders in a rare "all-party conference" to forge unity in the latest showdown with Washington.

US officials want Pakistan to launch an offensive against the Haqqanis, but the military says it is too over-stretched fighting local Taliban to open a new front against a US enemy that does not pose a threat to Pakistan.