US officials were holding talks with their Pakistani counterparts to resolve the issue, the Pentagon said, and US Senator John Kerry said he had spoken to Pakistan Prime Minister Yousuf Raza Gilani.

NATO said aircraft entered Pakistani airspace early Thursday in self-defence and killed "several armed individuals" after the crews believed they had been fired at from the ground.

The Pakistani government said it was investigating the incident in the Kurram district of Pakistan's northwestern tribal belt, which Washington has branded an Al-Qaeda headquarters and hub of militants fighting in Afghanistan.

The region is being targeted by a record number of US drone strikes and was reportedly where Al-Qaeda hatched a plot to attack cities in Britain, France and Germany uncovered by Western intelligence agencies.

AFP