Pakistan shut down NATO supply routes into Afghanistan - used for sending in nearly half of the alliance's land shipments - in retaliation for the worst such incident since Islamabad uneasily allied itself with Washington following the September 11, 2001 attacks on the United States.

Islamabad also said it had ordered the United States to vacate a drone base in the country, but a senior U.S. official said Washington had received no such request and noted that Pakistan had made similar eviction threats in the past, without following through.

NATO and U.S. officials expressed regret about the deaths of the Pakistani soldiers, indicating the attack may have been an error; but the exact circumstances remained unclear.

"Senior U.S. civilian and military officials have been in touch with their Pakistani counterparts from Islamabad, Kabul and Washington to express our condolences, our desire to work together to determine what took place, and our commitment to the U.S.-Pakistan partnership which advances our shared interests, including fighting terrorism in the region," said White House national security council spokesman Tommy Vieter.