KABUL, Afghanistan — Pakistan closed down a critical supply route for U.S. and allied forces in Afghanistan on Thursday after U.S. helicopters crossed into Pakistan during a confused, predawn attack that killed three Pakistani paramilitary troops.
Pakistan closed one of the two main crossings into Afghanistan hours after a pair of Apache helicopters apparently attacked a border post, manned by the paramilitary Frontier Corps, about 200 yards inside Pakistan.
U.S. military officials said that the helicopters opened fire in self-defense after taking small-arms fire from unknown forces inside Pakistan.
Hundreds of supply trucks bound for the busy Torkham crossing north of Peshawar were sidelined in Pakistan as the U.S.-led International Security Assistance Force said it was investigating. Pakistan's other main route into landlocked Afghanistan, in Chaman in the southeast, stayed open.
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The incident immediately raised tensions between the uneasy allies even as CIA chief Leon Panetta was conferring with Pakistan President Asif Ali Zardari.
"The government of Pakistan strongly disapproves any incident of violation of its sovereignty," Zardari told the CIA chief, according to a statement from the president's office. "Any violation of internationally agreed principles is counter-productive and unacceptable."
Without directly accepting responsibility for killing the Frontiers Corps forces, ISAF issued a statement conveying "sincere condolences to the Pakistani military and the families of those who were killed or injured."
ISAF acknowledged that two Apache helicopters "briefly" crossed into Pakistani airspace.
"After the initial strike, the aircraft received what the crews assessed as effective small-arms fire from individuals just across the border in Pakistan," ISAF said. "Operating in self defense, the... aircraft entered into Pakistani airspace killing several armed individuals."