The attacks took place in Mandi Khel, 25 kilometres (15 miles) north of Miranshah, the main town of North Waziristan tribal agency along the Afghan border and a known hub of the al-Qaeda-linked Haqqani network.

The organisation, created by Afghan warlord Jalaluddin Haqqani, fights foreign troops in Afghanistan from bases in North Waziristan.

Seven militants were killed in the first attack when US drones fired four missiles and destroyed a car and a militant compound, officials said.

"Two US drones fired four missiles. At least seven militants have been killed," one security official in Miranshah told AFP.

"Three militants were killed in the car while four were killed in the house," a security official in Peshawar told AFP.

In the second attack, a US drone fired two missiles and killed four militants, officials said.

A security official in Peshawar said militants had gathered to rescue the injured and remove the dead from the first attack when they were hit by the US drone.

The 11 men who died were thought to be attached to Taliban commander Hafiz Gul Bahadur, security officials in Miranshah and Peshawar told AFP.

One security official at Miranshah said they are seeking more information as informants in the area suspect there were foreigners among the dead.

"We have received such reports that four foreigners were also killed in these fresh attacks but their identity are still not known and we are collecting more information," the official said.

The US does not confirm drone attacks but its military and the CIA operating in Afghanistan are the only forces that deploy the unmanned aircraft in the region.

The covert campaign last year doubled missile attacks in the tribal area, where more than 100 drone strikes killed 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.

The US strikes are deeply unpopular among the Pakistani public, who see military action on Pakistani soil as a breach of national sovereignty and say some attacks have killed innocent civilians.

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