Militants attacked NATO supply trucks in northwest Pakistan with guns and mortars on Monday, setting vehicles ablaze and killing at least six people, officials said.
The attack took place in Khyber district, one of seven tribal areas on the Afghan border that are a haven for Taliban and Al-Qaeda linked insurgents.
Up to 20 armed militants targeted at least three Afghanistan-bound vehicles loaded with military equipment, said senior administration official Jehangir Azam.
"At least three NATO vehicles caught fire, four people died on the spot and two wounded in the attack expired later in hospital," Azam told AFP.
Another official , Asmatullah Wazir, said four NATO vehicles were hit and three of them caught fire in the attack in the Shagai area, 20 kilometres (12 miles) southeast of Landi Kotal, the district's main town.
The victims were Pakistani drivers and their helpers, he said.
A military official in the main northwestern city of Peshawar confirmed the militants used rockets and automatic weapons.
Pakistan is a key transit route for the NATO mission in landlocked Afghanistan. Supplies are driven across the border from the Arabian Sea port of Karachi.
From November 2011 to July 2012, Pakistan shut its Afghan border to overland NATO traffic after botched US air raids killed 24 Pakistani troops.
Pakistan and the United States have signed a deal allowing NATO convoys to travel into Afghanistan until the end of 2015.