US drone strikes targeted militant vehicles in Pakistan's northwestern tribal belt on Sunday, killing nine Islamist fighters, security officials said.

Two missiles were fired at a vehicle soon after it left a militant compound near Miranshah, the main town in the North Waziristan province, a security official said.

"The attack blew up the vehicle and also damaged the compound. Five militants were killed, they had been inside the vehicle, which was destroyed in the attack," the official said from Miranshah.

Another official, in the city of Peshawar, said the pilotless aircraft fired four missiles in two separate raids on the car in Dandey Saidgai village, confirming the deaths of five militants.

A drone then fired two missiles at a car in Moizer town, 40 kilometres (25 miles) west of Miranshah, officials said.

"Four militants were killed as the attack blew up their car in a fireball," an official said.

Two other other security officials confirmed the attack and the casualties.

The region is considered a fortress for Taliban and Al-Qaeda-linked fighters and the focus of a dramatic increase in US drone strikes.

It was not immediately clear if the drones were chasing any high value target, officials said.

A covert US drone campaign in Pakistan has stepped up strikes in the tribal belt, as intelligence claims emerged last month of a Mumbai-style terror plot to launch commando attacks on European cities.

The latest attacks came after a rare triple drone strike on Wednesday killed at least 11 militants in the same region.

The United States considers Pakistan's tribal belt an Al-Qaeda headquarters and the most dangerous place on Earth.

More than 200 people have been killed in over 40 strikes since September 3, heightening tensions with Islamabad over reported US criticism of Pakistan's failure so far to launch a ground offensive in North Waziristan.

Sunday's attacks came as US President Barack Obama was visiting neighbouring India.

The drone campaign is seen as integral to US-led efforts to turn around a nine-year Taliban insurgency in neighbouring Afghanistan.

Afghan and US officials say that militants use rear bases in Pakistan to orchestrate attacks in Afghanistan.

The war in Afghanistan is now at its deadliest, killing at least 626 foreign soldiers this year and thousands of civilians since the 2001 US-led invasion brought down the Taliban regime.

As a result, there is growing recognition that a military victory is not possible and there have been increasing reports of reconciliation efforts.

Officials in Washington say drone strikes have killed a number of high-value targets, including the Pakistani Taliban's founding father Baitullah Mehsud and a brother of Afghan warlord Sirajuddin Haqqani, who runs the Haqqani network.

But in Pakistan, anger over the attacks has fuelled reprisals from militant groups who have targeted NATO supply convoys destined for Afghanistan.

Militants publicly executed three tribesmen in the tribal belt on Sunday after accusing them of spying for the United States, officials said.

The men, blindfolded with hands tied behind their backs, were lined up near a gas station near the Afghan border, local police official Nasir Khan said.

"A man wearing a black mask then shot them dead one by one," he said, adding that the militants, before killing the trio, asked local people to "come and witness the fate of US spies".

Militants frequently kidnap and kill tribesmen in the troubled region, accusing them of spying for the Pakistani government or US forces in Afghanistan.