Two US drone strikes killed 15 militants on Saturday in the lawless tribal belt in Pakistan, where a main land route for NATO supplies was blocked for a third consecutive day.

Officials in Washington say drone strikes have killed several high-value targets, including Pakistani Taliban chief Baitullah Mehsud, and help protect troops in Afghanistan from attacks plotted across the border.

However, drone attacks are a sensitive issue in Pakistan and fuel anti-American sentiment in the conservative Muslim country.

In the latest strikes, US missiles hit two villages near Datta Khel town, about 45 kilometres (30 miles) west of Miranshah, the main town in North Waziristan tribal district.

In the first attack, four missiles hit a house used by militants in Dashgah village, killing six people.

"Two US drones fired four missiles and destroyed the house. Six militants were killed in this attack," a Pakistani security official in Peshawar told AFP on condition of anonymity.

"All of them were militants attached to the Haqqani group," one intelligence official said, referring to one of the strongest US foes in Afghanistan, whose leadership is based in North Waziristan.

A second intelligence official told AFP that initial reports suggested the dead were Uzbek militants from Afghanistan.

The second attack hit Inzarkas village, around two kilometres (one miles) from the first, killing nine militants.

"Two missiles hit a vehicle, while another two hit a house, a total of nine militants were killed," one intelligence official said.

"Latest reports said the US drone fired four missiles and targeted a vehicle and a house," another intelligence official in Miranshah said.

A security official in Peshawar also confirmed the death toll in the second strike.

Pakistani officials have reported that more than 20 US drone strikes in September killed around 120 people, the highest monthly toll for the attacks.

The overwhelming majority have been carried out in North Waziristan, considered a bastion of Al-Qaeda-linked forces and Taliban commanders opposed to the US-led war in Afghanistan.

Washington has classified Pakistan's tribal belt on the Afghan border a global headquarters of Al-Qaeda and the most dangerous place on Earth.

Under US pressure to crack down on Islamist havens, Pakistan has stepped up its own military operations against militants in the area.

But commanders have so far avoided a major offensive in North Waziristan, arguing that gains elsewhere need to be consolidated to prevent their troops from being stretched too thin.

Pakistan has protested against the drone strikes but never taken any practical steps to stop the US carrying out such attacks.

However, Pakistan's move this week to block a main land route for NATO supplies heading into Afghanistan has shown its determination to protect its airspace from US aircraft.

Pakistan halted the NATO convoys on Thursday after officials blamed a cross-border NATO helicopter attack for the deaths of three Pakistani soldiers. On Saturday the route remained closed.

"The situation is the same. The border is blocked for NATO supplies," an official at Torkham, the main border crossing in Pakistan's Khyber district, told AFP by telephone.

An official in Peshawar also confirmed that convoys had been suspended for a third day but said negotiations were continuing to reopen the route.

The Khyber pass at Torkham is on one of the main NATO supply routes through Pakistan into Afghanistan, where more than 152,000 US and NATO forces are fighting a Taliban-led insurgency.

NATO said its aircraft had entered Pakistani airspace Thursday in self-defence and killed "several armed individuals" after the air crews believed they had been fired upon.

It was the fourth such strike this week by NATO helicopters pursuing militants into Pakistan, in actions that have been condemned by Islamabad.

Pakistan has insisted that it has not agreed to any "hot pursuit" policy, which would allow incursions into its territory from Afghanistan.

"Any impression to the contrary is not factually correct. Such violations are unacceptable," a foreign ministry statement said.