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

US Secretary of State Hillary Clinton and Pakistani Foreign Minister Hina Rabbani Khar spoke by telephone, as did General Martin Dempsey, the chairman of the Joint Chiefs of Staff, and Pakistani Chief of Army Staff General Ashfaq Parvez Kayani.

The NATO-led force in Afghanistan confirmed that NATO aircraft had probably killed Pakistani soldiers in an area close to the Afghan-Pakistani border.

Reuters