Wednesday's attack was the fourth suspected drone strike in northwest Pakistan since Monday evening, as the US attempts to deal with fighters targeting foreign troops in neighbouring Afghanistan.

Many al-Qaeda and Taliban members fled to northwestern Pakistan following the collapse of Afghanistan's Taliban government in 2001, and are believed to orchestrate cross-border attacks from there.  

On Tuesday, two attacks by unmanned aircraft in South and North Waziristan killed around 12 fighters.

The US military does not confirm drone attacks, but its militatry and the Central Intelligence Agency (CIA) operating in Afghanistan are the only forces in the region that use the pilotless aircraft.

There have been about 60 such strikes since the beginning of 2008, including one in early August that killed Baitullah Mehsud, the leader of the Pakistani Taliban.

About 510 people, most of them believed to pro-Taliban fighters, have been killed in the strikes since early last year, according to a tally of reports from Pakistani security officials and residents.