The U.S. ambassador to Pakistan has apologized for a recent helicopter attack that killed Pakistani soldiers near the Afghan border.
Anne Patterson said in a statement Wednesday that a joint investigation has established that U.S. helicopters mistook the soldiers for insurgents they had been pursuing.
Pakistan has said the attack on Sept. 30 killed three members of Pakistan's Frontier Corps and wounded three others.
But Patterson's statement said two soldiers were killed and four were injured.
Following the attack, Pakistan shut down a key border crossing used to ship a large proportion of goods to NATO soldiers in Afghanistan.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
QUETTA, Pakistan (AP) -- Gunmen torched more than two dozen tankers carrying fuel to NATO troops and killed a driver Wednesday, 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 fabled 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 that has remained open.
In Washington, Pentagon spokesman Geoff Morrell said an investigation of the helicopter attack was expected to be concluded later Wednesday, and that he expected the spat between allies could be resolved soon.
Tension between the two countries has also been fueled by a record number of U.S. drone strikes in recent weeks, including one Wednesday that killed six militants in the North Waziristan tribal area near the Afghan border.
The U.S. has played down the impact of Pakistan closing the Torkham crossing, saying it has supply routes through other countries into Afghanistan and the closure has not caused fuel problems for NATO troops.
"We don't suspect it will, even if this were to last into the future," said Morrell Tuesday at the Pentagon. "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 southwestern Pakistan and via Central Asian states.
Still, 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 U.S. troops from a base that was used to ferry supplies into Afghanistan, and last year Kyrgyzstan threatened to do the same, though it has since backed down.
The tanker attack early Wednesday 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 parking lot of a roadside hotel on the outskirts of Quetta, the capital of Baluchistan province. At least 25 trucks were destroyed by fire that 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 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 attack, but the Pakistani Taliban have claimed responsibility for similar assaults on NATO supplies, including one before dawn Monday that killed four people.
The events have exposed the frequent strains in the alliance between Pakistan and the United States, but 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."
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 the one Wednesday that killed six militants near Miran Shah, the main town in North Waziristan, according to two Pakistani intelligence officials.
At least three of the militants were from the Haqqani network, which U.S. officials have said poses the greatest threat to NATO troops in Afghanistan, said two other Pakistani intelligence officials.
The officials spoke on condition of anonymity because they were not authorized to talk to the media.
The U.S. carried out at least 21 drone strikes in Pakistan in September -- almost double the previous monthly record. Washington does not publicly acknowledge the strikes, but U.S. officials have said privately that they have killed several senior Taliban and al-Qaida commanders.
Associated Press writers Riaz Khan in Peshawar, Ishtiaq Mahsud in Dera Ismail Khan, Rasool Dawar in Islamabad and Anne Gearan in Washington, D.C., contributed to this report.