A salvo of US missiles targeted Islamist fighters in Pakistan's tribal belt on Thursday, killing six militants returning from an insurgent bastion in Afghanistan, security officials said.

Six missiles slammed into the group in North Waziristan, the northwestern district considered the heartland of Islamist militants in Pakistan and the focus of a dramatic increase in US drone strikes since early September.

The missiles targeted the group about 12 kilometres (eight miles) from North Waziristan's main town of Miranshah, in an area that borders Khost in eastern Afghanistan, a hotbed of insurgents fighting US-led troops.

"Several unmanned US aircraft fired six missiles targeting militants in Gulli Khel village near the Ghulam Khan district of North Waziristan," the official told AFP on condition of anonymity.

The official said the militants had been returning on foot from Afghanistan, although a local intelligence source in Miranshah said the US missiles targeted a compound of militants from the Haqqani network.

The Haqqani network is linked to the Taliban and Al-Qaeda. Its leadership is based in North Waziristan and has been accused of plotting some of the deadliest attacks on US troops in Afghanistan, including a suicide bombing that killed seven CIA operatives at a US base in Khost last December.

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

Thursday's strike was the second this week, after a similar operation killed at least nine suspected Islamist fighters near Miranshah on Sunday.

Around 220 people have been killed in more than 40 strikes since September 3, straining relations with Islamabad already tense over reported US criticism of Pakistan's failure so far to launch a ground offensive in North Waziristan.

The covert US drone campaign in Pakistan dramatically increased in September as intelligence claims emerged of a Mumbai-style terror plot to launch commando attacks in European cities.

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

More than 150,000 foreign troops are battling footsoldiers whose leaders are widely believed to be based in Pakistan.

Afghan and US officials allege that militants with rear bases in Pakistan enjoy at least some measure of protection from the Pakistani intelligence services — charges that Islamabad denies.

During a recent visit to India, US President Barack Obama said Islamabad was making progress on tackling what he called the "cancer" of extremism but it was not happening quickly enough.

The war in Afghanistan is now at its deadliest, killing at least 633 foreign soldiers this year and thousands of civilians since the 2001 US-led invasion brought down the Taliban regime.

A senior US commander told AFP on Thursday that operations are being stepped up around the southern Afghan Taliban flashpoint of Kandahar to guard against attacks before the Muslim festival of Eid and a key NATO conference.

"We think that the Taliban are going to try and stage an explosion or series of explosions at Eid to create a media buzz to give the perception of insecurity," Lieutenant Colonel Victor Garcia said.

The NATO alliance meets in Lisbon next week with discussions likely to be dominated by a timetable for the handover of security to Afghan forces.

Officials in Washington say drone strikes have killed a number of high-value targets, but in Pakistan anger over the operations has fuelled reprisals from militant groups who have attacked NATO supply convoys destined for Afghanistan.

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