US missile attacks have killed at least 13 militants, four of them foreigners, in Pakistan's most notorious extremist bastion hugging the Afghan border, officials said Thursday.

US missiles flattened a fort used as a Taliban training centre in Sanzali village of North Waziristan on Wednesday in the fourth suspected US drone attack in the tribal district in a week.

Just over an hour later, a suspected drone slammed another missile into a group of militants sifting through the wreckage, searching for survivors and picking out the dead bodies.

"We can now confirm that 13 militants were killed in the two strikes," a senior security official told AFP.

Another official said four of the dead were foreigners. Their nationalities are not yet known. Arab, Uzbek and Afghan militants are present in the area.

Suspected US drones have increasingly hit North Waziristan, a bastion of Al-Qaeda fighters, the Taliban and the Haqqani network that attacks the 113,000 US and NATO troops fighting in neighbouring Afghanistan.

The district neighbours South Waziristan, where Pakistan has been focusing its most ambitious offensive yet against homegrown Taliban militants. It sent about 30,000 troops into the region on October 17.

At least 74 US drone missile strikes have killed more than 680 people in Pakistan since August 2008.

The United States does not confirm drone attacks, but its military is the only force that deploys combat drones in the region.