Gunmen torched more than two dozen tankers carrying fuel to Nato troops and killed a driver today - 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 in the border area 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 of the helicopter attack was expected to be concluded later today, and that he expected that the spat between allies could be resolved soon.
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.
"We don't suspect it will, even if this were to last into the future," he said yesterday. "But we really do have a sense we're making progress and this can be resolved soon."
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.
Uzbekistan evicted US troops from a base which was used to ferry supplies into Afghanistan, and last year Kyrgyzstan threatened to do the same, though it has since backed down.
The attack early this morning came on trucks on their way to the Chaman crossing.
An unidentified number of gunmen in two vehicles attacked the trucks as they sat 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.
Of the six attacks on convoys bringing supplies in from the port city of Karachi since the Torkham closure, four were on trucks heading to that crossing and two were on their way to Chaman.
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.
The events have exposed the frequent strains in the alliance between Pakistan and the United States, but Mr Morrell played down the possibility of any lasting effects.
"There are incidents which create misunderstandings, there are setbacks, but that does not mean the relationship - this crucial relationship to us - is in any way derailed."
In addition to ensuring safe passage for Nato supplies, the US needs Pakistan to help target Taliban and al Qaida militants who stage cross-border attacks against foreign troops in Afghanistan. In return, Pakistan receives billions of dollars in military and civilian assistance that help keep its economy afloat.
Even if the border is reopened, underlying tensions will remain in the US-Pakistan relationship, especially over Pakistan's unwillingness to go after Afghan Taliban militants on its territory with whom it has strong historical ties and who generally focus their attacks on Western troops, not Pakistani targets.
The US has responded by dramatically increasing the number of CIA drone strikes in Pakistan's tribal belt, carrying out 21 suspected attacks in September - nearly double the previous monthly record.
The US has also stepped up military operations along the Afghan border, but officials in Washington said the recent Nato cross-border helicopter strikes were not a strong-arm tactic aimed at pressuring Pakistan.
The officials said the US did not oppose the temporary closure of Torkham because it lets Pakistan rebuke Washington in a way which plays well to the domestic Pakistani audience without seriously hampering US military operations.Reuse content