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One driver was killed in the attack and at least 25 trucks were destroyed by a fire that spread quickly from vehicle to vehicle, senior police official Hamid Shakil said.

On Wednesday night, suspected militants armed with assault rifles opened fire on oil tankers parked along the road in Khyber Pakhtunkhwa province as they were making their way to Torkham. At least 30 tankers were engulfed in flames, said local police Officer Nisar Khan. It was unclear if there were any casualties.

Of the seven attacks on convoys bringing supplies in from the port city of Karachi since the Torkham closure, five were on trucks heading to that crossing and two were on their way to Chaman.

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

It was unclear who was behind the latest attacks, but the Pakistani Taliban have claimed responsibility for similar assaults on NATO supplies.

The helicopter attack and the border closure have exposed the frequent strains in the alliance between Pakistan and the United States. But Pentagon spokesman Geoff Morrell downplayed 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,” Mr. Morrell said Tuesday.

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 and who generally focus their attacks on Western troops, not Pakistani targets.

The U.S. has responded by dramatically increasing the number of CIA drone strikes in Pakistan’s tribal belt, including two Wednesday that killed 11 militants in North Waziristan, according to Pakistani intelligence officials speaking on condition of anonymity because they were not authorized to talk to the media.

In the first attack, a U.S. drone fired two missiles at a house near Miran Shah, the main town in North Waziristan, killing six militants, the officials said.

About two hours later, missiles struck a house near Mir Ali, another major town in North Waziristan, killing five militants, the officials said.

The United States does not publicly acknowledge the drone strikes in Pakistan, but U.S. officials have said privately that they have killed several senior Taliban and al Qaeda commanders.

Associated Press writers Riaz Khan in Peshawar, Pakistan; Ishtiaq Mahsud in Dera Ismail Khan, Pakistan; Rasool Dawar in Islamabad; Abdul Sattar in Quetta, Pakistan, and Anne Gearan in Washington contributed to this report.