Reuters reports: 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, 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.

The attack is the worst single incident of its kind since Pakistan uneasily allied itself with Washington in the days immediately following the Sept 11, 2001 attacks on U.S. targets.

Relations between the United States and Pakistan, its ally in the war on militancy, have been strained following the killing of al Qaeda leader Osama bin Laden by U.S. 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 Prime Minister Yusuf Raza Gilani. “We will not let any harm come to Pakistan’s sovereignty and solidarity.”