U.S. drone aircraft fired missiles in Pakistan's North Waziristan tribal region on Monday, killing at least four militants, intelligence officials and tribesmen said, the latest strikes in a resumption of the air campaign.

The unacknowledged Central Intelligence Agency drone programme, a key element in President Barack Obama's counter-terrorism strategy, was seemingly paused after a November 26 NATO cross-border air attack killed 24 Pakistani soldiers, triggering fury. The United States resumed the programme on January 10.

In Monday's attacks, two missiles hit a vehicle in the village of Degan, and another two struck a house in nearby Mohammedkhel village.

Intelligence officials said the strike on the vehicle killed mostly Turkmen, who were possibly members of al Qaeda. There may be more casualties, the officials added.

The use of unmanned armed aircraft over Pakistan has been a sore point with the public and Pakistani politicians, who describe them as violations of sovereignty that produce unacceptable civilian casualties.

But despite its public stance, Pakistan has quietly supported the drone programme since Obama ramped up air strikes after taking office in 2009, and even asked for more flights.

Several militant groups have strongholds in Pakistan's northwestern tribal regions, taking advantage of the porous border with Afghanistan to conduct cross-border attacks.

A Pakistan security source told Reuters that the Jan 10 strike, which targeted senior al Qaeda leader Aslam Awan, and a follow-up attack two days later, were joint operations.

There are unconfirmed reports that Hakimullah Mehsud, leader of the Pakistani Taliban, the militant group that poses the gravest security threat to the country, was killed in the January 12 drone strike, but many in the American and Pakistan intelligence communities doubt this.

Relations between Islamabad and Washington plunged to their lowest level in years after the November 26 NATO attack next to the Afghanistan border, and prompted Pakistan to put its ties with the United States on hold for a review.

(Additional reporting by Saud Mehsud in DERA ISMAIL KHAN and Jibran Ahmad in PESHAWAR; Writing by Qasim Nauman; Editing by Chris Allbritton; Editing by Sanjeev Miglani)