A small bomb in north-western Pakistan has damaged a truck which was carrying oil to Nato troops in Afghanistan - the latest attack on stalled supply convoys since Pakistan shut a key border crossing to international forces last week.
The attacks have raised tensions which were already heightened by Pakistan's decision to close the Torkham crossing in apparent reaction to a series of alleged Nato helicopter attacks on Pakistani territory, including one which killed three Pakistani soldiers.
The bomb that exploded was placed underneath the tanker while it was parked alongside more than 100 other trucks waiting to cross into Afghanistan, said Wajid Khan, a local administrator in the Khyber tribal region where the border crossing is located.
There were no casualties, but the risk of fire was high since oil was leaking out of the damaged tanker, said Mr Khan. Authorities moved other trucks away from the vehicle as a precaution, he added.
It was unclear who was behind the bombing, but the Pakistani Taliban has claimed responsibility for similar attacks on Nato supply convoys, including one before dawn on Monday in which four people were killed.
There have been at least five attacks on supply convoys since the Torkham closure - four were heading to that crossing and one was en route to another crossing which has remained open.
The events of the last week have exposed the often strained nature of the alliance between Pakistan and the United States. But analysts doubt it will reach a breaking point because each side is so reliant on each other.
In addition to 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 - although Islamabad is not always the most co-operative partner.
In return, Pakistan receives billions of dollars in military and civilian assistance which help keep its economy afloat.