"It is also not yet clear if there was any high value target present in the area at the time of attack," he added.

Another intelligence official said there were five foreigners among the slain fighters, but their nationalities could not be immediately confirmed.

The United States has intensified missile strikes using pilotless drone aircrafts
in Pakistan's northwest tribal in an effort to halt violence in Afghanistan, which US officials say comes from fighters on the Pakistani side.

US drone strikes are unpopular in Pakistan and the government in Islamabad has repeatedly urged Washington to end them.

Pakistan says the attacks kill civilians, stoking resentment among the local population and thus proving to be counter-productive in the battle against armed groups.

The attacks also undermine Pakistani sovereignty, Islamabad says.

But the US has persisted with the drone attacks, saying they are effective means to deal with Taliban and al-Qaeda fighters operating in Pakistan's border areas with Afghanistan.