The Pakistani foreign minister Sunday described a NATO attack that killed dozens of soldiers as “totally unacceptable,” saying it plunges the nation’s relationship with the United States into deeper crisis.

NATO helicopters attacked a military checkpoint in northwest Pakistan on Saturday, killing 24 Pakistani soldiers, authorities in the nation said.

The attack wounded 13 others in Mohmand Agency, one of seven districts in the volatile region bordering Afghanistan, the Pakistani foreign ministry said in a statement.

Many of the wounded were in critical condition, military officials said. The officials did not want to be identified because they are not allowed to talk to the media.

The NATO secretary general said it was a “tragic unintended” incident, and pledged to ensure such attacks don’t reoccur.

“NATO remains strongly committed to work with Pakistan to improve cooperation to avoid such tragedies in the future,” Anders Fogh Rasmussen said in a statement.

But the Pakistani foreign minister said the attack violated the nation’s sovereignty.

“Such attacks … demonstrate complete disregard for international law and human life,” Foreign Minister Hina Rabbani Khar told U.S. Secretary of State Hillary Clinton in a phone call early Sunday.

The minister’s office issued a news release describing the conversation.

The attack “negates the progress made by the two countries on improving relations and forces Pakistan to revisit the terms of engagement,” Khar said.

Pakistan shut down two NATO supply routes in response to the attack, military and intelligence officials said.

NATO trucks have used the routes, in Khyber Agency and Balochistan, to supply U.S. and international forces fighting in Afghanistan.

The foreign minister briefed Clinton on decisions to stop the supply routes, and asked the U.S. to vacate the Shamsi air base within 15 days.

NATO and U.S. officials expressed their condolences about the deaths of the Pakistani soldiers, but circumstances surrounding what happened remained unclear.

Pakistan said it will reassess its relationship with the United States, NATO and the International Security Assistance Force, marking a major setback in worsening U.S.-Pakistan relations.

“The prime minister will take the Parliament into confidence on the whole range of measures regarding matters relating to Pakistan’s future cooperation with US/NATO/ISAF, in the near future,” Pakistani Prime Minister Yousuf Raza Gilani’s office said in a statement.

The incident could be the deadliest for Pakistani soldiers involving NATO since a U.S. airstrike in June 2008, which Pakistan said killed 11 of its forces who were cooperating with the U.S.-led war in Afghanistan.

That airstrike, also in Mohmand Agency, prompted Islamabad to summon the U.S. ambassador and lodge an official protest.