A US drone attack on a militant compound in Pakistan's northwestern tribal area killed six insurgents on Sunday, security officials said.

The unmanned aircraft fired two missiles on the compound in Shawal district, 50 kilometres (30 miles) southwest of Miranshah, the main town of North Waziristan tribal district, near the Afghan border, they said.

"Two missiles targeted the compound, killing six militants," a security official told AFP.

"The strike destroyed the house and triggered a fire," another official said.

"It was difficult to identify the bodies immediately as some of them were charred," he said.

Washington considers Pakistan's semi-autonomous northwestern tribal belt the main hub of Taliban and Al-Qaeda militants plotting attacks on the West and in Afghanistan.

A similar attack in the region on Tuesday killed five militants.

There has been a dramatic increase in US drone strikes in Pakistan since May when a NATO summit in Chicago could not strike a deal to end a six-month blockade on NATO supplies crossing into Afghanistan.

A drone attack on June 4 killed 15 militants in North Waziristan, including senior Al-Qaeda figure Abu Yahya al-Libi.

Islamabad is understood to have approved the drone strikes on Al-Qaeda and Taliban targets in the past. But the government has become increasingly vocal in its public opposition as relations with Washington have nosedived.

Both sides are at loggerheads over reopening NATO supply lines that Pakistan shut in fury on November 26 when US air strikes killed 24 Pakistani soldiers.

An impasse continues as Islamabad insists on an American apology for their deaths and an end to drone strikes.

US officials consider the attacks a vital weapon in the war against Islamist extremists, despite concerns from rights activists over civilian casualties.

The London-based Bureau of Investigative Journalism has said that under President Barack Obama one drone strike has hit Pakistan on average every four days.

It said most of the 2,292 to 2,863 people reported to have died were low-ranking militants, but that only 126 fighters had been named.

It said it had credible reports of between 385 and 775 civilians being killed, including 164 to 168 children.

UN High Commissioner for Human Rights Navi Pillay last month called for a UN investigation into US drone strikes in Pakistan, questioning their legality and saying they kill innocent civilians.

The UN human rights chief provided no statistics but called for an investigation into civilian casualties, which she said were difficult to track.

She said UN chief Ban Ki-moon had urged states to be "more transparent" about circumstances in which drones are used and take necessary precautions to ensure that the attacks involving drones comply with applicable international law.