NATO helicopters and fighter jets attacked two military outposts in northwest Pakistan on Saturday, killing as many as 28 troops and plunging U.S.-Pakistan relations deeper into crisis.
Pakistan retaliated by shutting down NATO supply routes into Afghanistan, used for sending in nearly half of the alliance's land shipments. It also said it would ask U.S. forces to quit an air base used for CIA drone strikes on militants.
The attack is the worst incident of its kind since Pakistan uneasily allied itself with Washington following the Sept. 11, 2001 attacks on the United States.
The NATO-led force in Afghanistan confirmed that NATO aircraft had probably killed Pakistani soldiers in an area close to the Afghan-Pakistani border.
"Close air support was called in, in the development of the tactical situation, and it is what highly likely caused the Pakistan casualties," said General Carsten Jacobson, spokesman for the International Security Assistance Force (ISAF).
He added he could not confirm the number of casualties, but ISAF was investigating. "We are aware that Pakistani soldiers perished. We don't know the size, the magnitude," he said.
Pakistan's Prime Minister Yusuf Raza Gilani said the killings were "an attack on Pakistan's sovereignty", adding: "We will not let any harm come to Pakistan's sovereignty and solidarity."
The Foreign Office said it would take up the matter "in the strongest terms" with NATO and the United States, while the Chief of Army Staff, General Ashfaq Pervez Kayani, said steps would be taken to respond "to this irresponsible act".
"A strong protest has been launched with NATO/ISAF in which it has been demanded that strong and urgent action be taken against those responsible for this aggression."
Two military officials said up to 28 troops had been killed and 11 wounded in the attack on the outposts, about 2.5 km (1.5 miles) from the Afghan border. The Pakistani military said 24 troops were killed and 13 wounded.