Police say gunmen in Pakistan have torched four tankers carrying fuel for NATO forces in Afghanistan. The attackers also killed a truck driver.

Wednesday's assault was the latest on supply convoys heading for Afghanistan since Pakistan shut a key border crossing to international forces last week.

Police official Shah Nawaz Khan says the attack occurred in the parking area of a roadside hotel on the outskirts of the southwest city of Quetta.

The attacks have raised tensions already elevated by Pakistan's decision to close the Torkham crossing in apparent reaction to alleged NATO helicopter strikes on its territory. One of those strikes killed three Pakistani soldiers.

The tankers hit Wednesday were believed headed for a smaller border crossing that remains open.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

PESHAWAR, Pakistan (AP) _ A small bomb damaged a truck in northwestern Pakistan on Tuesday that 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 that were already elevated 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 that killed three Pakistani soldiers.

The bomb that exploded Tuesday was placed underneath the tanker while it was parked in a lot 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.

The attack didn't result in any casualties, but the risk of fire was high since oil was leaking out of the damaged tanker, said Khan. Authorities moved other trucks away from the vehicle as a precaution, he said.

It was unclear who was behind the bombing, but the Pakistani Taliban have claimed responsibility for similar attacks on NATO supply convoys, including one before dawn Monday that killed four people.

There have been at least five attacks on the supply convoys since the Torkham closure _ four of them that were heading to that crossing and one that was headed to another crossing that 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 U.S. 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.

Both American and Pakistani officials have predicted the Torkham border crossing will reopen within a few days.

NATO's Secretary-General Anders Fogh Rasmussen tried to reduce the tension between the two sides Monday by apologizing for last week's helicopter attack that killed Pakistani troops, saying the casualties were "unintended" and that a joint investigation was under way.

But even if the border is reopened, underlying tensions will remain in the U.S.-Pakistan relationship, especially over Pakistan's unwillingness to go after Afghan Taliban militants on its territory with whom it has strong historical ties.

The U.S. has responded to Pakistan's intransigence by dramatically increasing the number of CIA drone strikes in the country's tribal region. The U.S. carried out 21 such attacks in September _ nearly double the previous monthly record.

The U.S. has also stepped up military operations along the Afghan border, but officials in Washington said the recent NATO helicopter strikes inside Pakistan were not a strong-arm tactic meant to show the country that if it fails to vigorously target militants the U.S. will step in.

The officials said the temporary closure of the Torkham crossing was not opposed by U.S. officials because it allows Pakistan to let off steam and give a public rebuke to Washington that plays well to the domestic Pakistani audience without seriously hampering U.S. military operations in Afghanistan. The officials spoke on condition because they were not authorized to talk to the media.

Since Pakistan closed Torkham, traffic has backed up at various points along the route from the southern border of Karachi. But coalition officials say the closing has not resulted in shortages since hundreds of trucks still cross into landlocked Afghanistan each day through Pakistan and via Central Asian states. However, the Central Asian routes are more expensive and less convenient than those through Pakistan.

The convoys bring fuel, military vehicles, spare parts, clothing and other non-lethal supplies for foreign troops.

____

Associated Press Writer Anne Gearen contributed to this report from Washington.