By SEBASTIAN ABBOT

Associated Press

ISLAMABAD — Pakistan on Saturday blocked vital supply routes for U.S.-led troops in Afghanistan and demanded Washington vacate a base used by American drones after coalition aircraft allegedly killed 24 Pakistani troops at two posts along a mountainous frontier that serves as a safe haven for militants.

The incident was a major blow to American efforts to rebuild an already tattered alliance vital to winding down the 10-year-old Afghan war. Islamabad called the bloodshed in one of its tribal areas a "grave infringement" of the country’s sovereignty, and it could make it even more difficult for the U.S. to enlist Pakistan’s help in pushing Afghan insurgents to engage in peace talks.

A NATO spokesman said it was likely that coalition airstrikes caused Pakistani casualties, but an investigation was being conducted to determine the details. If confirmed, it would be the deadliest friendly fire incident by NATO against Pakistani troops since the Afghan war began a decade ago.

A prolonged closure of Pakistan’s two Afghan border crossings to NATO supplies could cause serious problems for the coalition. The U.S., which is the largest member of the NATO force in Afghanistan, ships more than 30 percent of its non-lethal supplies through Pakistan. The coalition has alternative routes through Central Asia into northern Afghanistan, but they are costlier and less efficient.

Pakistan temporarily closed one of its Afghan crossings to NATO supplies last year after U.S. helicopters accidentally killed two Pakistani soldiers. Suspected militants took advantage of the impasse to launch attacks against stranded or rerouted trucks carrying NATO supplies. The government reopened the border after about 10 days when the U.S. apologized. NATO said at the time the relatively short closure did not significantly affect its ability to keep its troops supplied.

But the reported casualties are much greater this time, and the relationship between Pakistan and the U.S. has severely deteriorated over the last year, especially following the American raid that killed Osama bin Laden in a Pakistani garrison town in May. 

The government announced it closed its border crossings to NATO in a statement issued after an emergency meeting of the Cabinet’s defense committee chaired by Pakistani Prime Minister Yousuf Raza Gilani.

It also said that within 15 days the U.S. must vacate Shamsi Air Base, which is located in southwestern Baluchistan province. The U.S. uses the base to service drones that target al-Qaida and Taliban militants in Pakistan’s tribal region when they cannot return to their bases inside Afghanistan because of weather conditions or mechanical difficulty, said U.S. and Pakistani officials, speaking on condition of anonymity to discuss sensitive strategic matters.

The government also plans to review all diplomatic, military and intelligence cooperation with the U.S. and other NATO forces, according to the statement issued after the defense committee meeting.