Two missiles fired by the unmanned aircraft hit a house in the village of Khushali Turikhel, 40 kilometres (25 miles) east of Miranshah, the main town in the lawless North Waziristan tribal district, security officials told AFP.

"The US drone fired two missiles which hit a house. Two locals and four militants of central Asian origin have been killed," a Pakistani security official said.

The official based in Peshawar said militants were using the house as a compound, which was completely destroyed.

Two intelligence officials based in Miranshah confirmed the attack and the number of casualties, adding that three militants whose identities were not yet clear were wounded in the strike.

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.

North Waziristan is the headquarters of the Haqqani leadership and the main militant bastion in the semi-autonomous tribal belt.

The Haqqani network is considered the deadliest enemy of US troops in eastern Afghanistan. It was founded by Jalaluddin Haqqani and is run by his son, Sirajuddin, both designated "global terrorists" by Washington.

The United States blames it over some of the most spectacular attacks in Afghanistan, such as last week's 19-hour siege in Kabul and the 2009 killing of seven CIA agents, and accuses Pakistani spies of having ties to the group.

In an unprecedented condemnation of Pakistan the US military's top officer Admiral Mike Mullen said this week that the country's main intelligence agency the ISI was actively supporting Haqqani network militants.

Pakistan has reacted angrily to the US allegations, saying they are "not acceptable" and warning that Washington stands to lose a vital ally.

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

Around two dozen 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.

Pakistani-US relations sank to a new nadir after the unilateral American raid that killed bin Laden but in recent months had appeared to recover slightly.

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.

Despite serious disagreement, the US military had no intention of cutting off dialogue with Pakistan, press secretary George Little told reporters.

"The lines of communication with our Pakistani counterparts remain open," Little said. "This is a relationship that's complicated but essential."

The US military's top officer, Admiral Mike Mullen, on Thursday leveled harsh accusations against Pakistan at a Senate hearing, saying the country's intelligence agency was backing proxies in the Al-Qaeda-linked Haqqani network that target US troops in Afghanistan.

Pakistan angrily denied the allegations, saying that the United States could lose an ally over the rift.

Pakistan's army chief General Ashfaq Parvez Kayani said Friday the charges were "not based on facts."

In talks with Pakistan's government, the United States has presented solid evidence of ISI's links to the Haqqani militants, a US official, who spoke on condition of anonymity, told AFP.

Mullen made his views clear to General Kayani last Saturday in a meeting in Spain, his spokesman, Captain John Kirby, told reporters.

Mullen has spoken before about links between the Inter-Services Intelligence agency (ISI) and Islamist militants but his sharp comments on Thursday came after a surge in attacks on US troops and interests by the Haqqani network, Kirby said.

"It's gotten worse. Their activities have become more brazen, more aggressive, more lethal and the information has become more available that the these attacks have been supported or even encouraged by the ISI," he said.

Mullen has not spoken to Kayani since Thursday's congressional hearing, he added.

"We still want to pursue a working, productive partnership with the Pakistani military, and that in large measure depends on their willingness and their ability to disconnect themselves from extremist groups like the Haqqani network," Kirby said.

On Thursday, Mullen accused the Haqqanis -- with ISI backing -- of this month's truck bombing on a NATO base in Afghanistan that wounded 77 Americans; a 19-hour siege on the US embassy in Kabul; and a June attack on the InterContinental hotel in Kabul.

A CIA asset turned Al-Qaeda ally, the Haqqani network is probably the most dangerous faction in the Afghan Taliban. In the 1980s, the United States funneled arms and cash to the Haqqani faction to counter Soviet forces.