Gunmen have torched more than two dozen tankers carrying fuel to Nato troops and killed a driver - the sixth attack on convoys taking supplies to Afghanistan since Pakistan closed a key border crossing almost a week ago.
Islamabad shut down the Torkham crossing along the Khyber Pass last Thursday after a Nato helicopter attack near the border killed three Pakistani troops.
The closure has left hundreds of trucks stranded alongside the country's highways and bottlenecked traffic heading to the one route into Afghanistan from the south which has remained open.
In Washington, Pentagon spokesman Geoff Morrell said an investigation into the helicopter attack was expected to conclude soon, and that he expected the spat between allies could be resolved. The US has supply routes through other countries into Afghanistan, and Mr Morrell emphasised that the Torkham closing had not caused fuel problems for Nato troops.
Hundreds of supply trucks still cross into landlocked Afghanistan each day through the Chaman crossing in south-western Pakistan and via Central Asian states.
Pakistan is the fastest and cheapest way to get goods to Afghanistan, and trouble with other routes in the past makes it even more vital.
The attack came as trucks were on their way to the Chaman crossing. An unidentified number of gunmen in two vehicles attacked the convoy in the car park of a roadside hotel on the outskirts of Quetta, the capital of Baluchistan province.
At least 25 trucks were destroyed by fire which spread quickly from vehicle to vehicle, senior police official Hamid Shakil said.
The convoys carry fuel, military vehicles, spare parts, clothing and other non-lethal supplies for foreign troops in Afghanistan.
It was unclear who was behind the latest attack, but the Pakistani Taliban have claimed responsibility for similar assaults on Nato supplies, including one before dawn on Monday in which four people were killed.