Islamabad ? Pakistan closed the Khyber Pass supply route for U.S. and NATO troops in Afghanistan after a coalition helicopter attack mistakenly killed three Pakistani soldiers at a border post Thursday, raising tensions in a vital relationship for both Islamabad and Washington.

NATO said its helicopters entered Pakistani airspace and hit a target only after receiving ground fire. The alliance expressed condolences to the families of the soldiers and said both nations would investigate the incident.

A lengthy ban on supply trucks would place intense strain on the U.S.-Pakistani relationship and hurt the Afghan war effort. But that was seen as unlikely, as neither Islamabad nor Washington can afford a meltdown in ties at a crucial time in the 9-year-old war.