Pakistani officials say another tanker carrying fuel for troops in neighboring Afghanistan has been attacked in the country's northwest.

Officials say a bomb planted underneath the tanker exploded Tuesday near the Torkhum border crossing in the Khyber tribal region.  There were no reports of casualties.

There have been at least five attacks on NATO supply convoys since last week, with the Taliban claiming responsibility for two of the attacks.

Trucks remain lined up at Torkhum.  Pakistan shut the border crossing last Thursday, following a NATO cross-border raid that Pakistan says killed three of its soldiers in the Kurram region.  Pakistani officials say they closed the border crossing for security reasons.

Isobel Coleman with the Council on Foreign Relations discusses the situation in Pakistan:

Pentagon spokesman Geoff Morrell told reporters in Washington Tuesday that the United States is working with the Pakistani government to reopen the crossing as soon as possible.  Morrell reiterated that the closing and the attacks on NATO tankers have not affected the military's ability to resupply troops in Afghanistan.

NATO says its forces were pursuing militants out of self defense when helicopters crossed the border into Pakistan on Thursday. Pentagon spokesman Morrell said Tuesday that NATO plans to release the result of a joint investigation with Pakistani officials in the next day.

Recent NATO border incursions and a sharp increase in attacks by unmanned planes flying over Pakistan have raised tensions between the international forces and Islamabad.  

Morrell said Tuesday that there have been occasional setbacks in day-to-day relations between Pakistan and the United States, but that the military-to-military relationship has been the strongest it has ever been.

The latest suspected U.S. missile strike took place Monday in Pakistan's North Waziristan tribal region.  Pakistani officials say eight militants, including five German nationals, were killed.