NATO helicopters and fighter jets attacked two military outposts in northwest Pakistan on Saturday, killing as many as 28 troops and plunging US-Pakistan relations, already deeply frayed, further into crisis. Pakistan retaliated by shutting down vital NATO supply routes into Afghanistan, used for sending in almost half of the alliance's non-lethal materiel. Relations between the United States and Pakistan, its ally in the war on militancy, have been strained following the killing of Osama bin Laden by US special forces in a raid on the Pakistani garrison town of Abbottabad in May, which Pakistan called a flagrant violation of sovereignty. The Pakistani government and military brimmed with fury. This is an attack on Pakistan's sovereignty,” said PM Yusuf Raza Gilani. “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. The powerful Chief of Army Staff, General Ashfaq Pervez Kayani, said in a statement issued by the Pakistani military that “all necessary steps be under taken for an effective response 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.” The commander of NATO-led forces in Afghanistan, General John R. Allen, said he had offered his condolences to the family of any Pakistani soldiers who “may have been killed or injured” during an “incident” on the border. A spokesman for the force declined further comment on the nature of the “incident” and said an investigation was proceeding. It was not yet clear, he said, whether there had been deaths or injuries. The U.S. embassy in Islamabad also offered condolences. NATO supply trucks and fuel tankers bound for Afghanistan were stopped at Jamrud town in the Khyber tribal region near the city of Peshawar hours after the raid, officials said. “We have halted the supplies and some 40 tankers and trucks have been returned from the check post in Jamrud,” Mutahir Zeb, senior government official told. Another official said the supplies had been stopped for security reasons.