A series of unmanned US drone strikes in a lawless district of Pakistan on Sunday killed at least 13 militants, officials said, as tribesmen took to the streets to protest the aerial campaign.

The first two attacks took place in Datta Khel, some 40 kilometres (25 miles) west of North Waziristan's main town of Miranshah. The third was in Mando Khel, around 60 kilometres south of the town.

In the first strike two missiles were fired on a vehicle seconds after it stopped outside a house in Datta Khel, officials said.

"The US drone hit a car immediately after it parked outside a house. Four militants have been killed," an intelligence official in Miranshah told AFP, adding that the vehicle was destroyed and the house badly damaged.

"Preliminary reports said all of them were local militants but we are trying to get more information," he added.

A second intelligence official said the drone fired two missiles.

An intelligence official in Peshawar also confirmed the attack and the number of missiles, saying: "At least four militants have been killed."

Another US drone then fired two missiles at a motorcycle travelling in the same town, killing three militants, local officials said.

"Three militants have been killed in the second attack," a security official in Miranshah told AFP.

Another security official said the militants were riding a motorcycle when they were hit by the US drone.

It was not immediately clear if the drones were pursuing any high value target but one intelligence official in Miranshah said they were checking reports that a foreigner was among those killed in the second attack.

In the third strike, a vehicle parked inside a suspected militant compound was hit.

"A US drone was chasing a vehicle, it fired two missiles just after the car was parked. Six militants have been killed and three injured," an intelligence official in Miranshah said.

The area close to the Afghan border is a key battleground in the fight against the Taliban and Al-Qaeda.

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.

A series of strikes on January 1 killed at least 15 people and destroyed a Taliban compound, according to Pakistani officials, and has been followed by at least three more separate drone strikes this month.

In the town of Mir Ali, also in North Waziristan, some 1,800 tribesmen staged a demonstration on Sunday against the continuous drone strikes, witnesses said.

All the markets and bazaars remained closed with traffic suspended on the busy Bannu-Miranshah road, which runs through Mir Ali.

"The government should take immediate steps to stop drone attacks otherwise we will launch a protest movement and will march towards Islamabad," Sherzali Khan, a local tribal elder, told the protesting tribesmen, who were shouting slogans against the United States and CIA.

A similar rally was held on Friday in Miranshah against the drone strikes, which the tribesmen said were killing innocent people including women and children.

The US strikes are deeply unpopular 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 covert 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.

But it has stalled on launching a ground offensive in North Waziristan, saying its troops are overstretched.

Washington says the strikes have killed a number of high-value targets, including the former Pakistani Taliban chief Baitullah Mehsud.