Militants attacked NATO supply trucks in northwest Pakistan with guns and mortars, killing at least three drivers and wounding six others, officials said.
The attack took place in the Shagai area, 20 kilometres (12 miles) southeast of Landi Kotal, the main town of Khyber district, one of seven tribal areas on the Afghan border that are a haven for Taliban and Al-Qaeda linked insurgents.
Another administration official in Khyber confirmed the account.
Pakistan is a key transit route for the NATO mission in landlocked Afghanistan, from where supplies are driven to 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.