Pakistani officials say NATO helicopters from Afghanistan attacked two military checkpoints in northwestern Pakistan early Saturday, killing at least 26 troops and wounding 14 others.
Pakistan retaliated within hours by closing both of its border crossings into Afghanistan, effectively suspending NATO supply convoys through its country to its landlocked neighbor. Top Pakistani civilian and military leaders also are meeting late Saturday to discuss the incident.
A Pakistani military spokesman said the pre-dawn attack in Salala, a village in the restive Mohmand tribal area near the Afghan border, was "unprovoked and indiscriminate." Authorities say about 40 soldiers were stationed at the post.
NATO officials say they are aware of the incident and are investigating.
Top NATO and U.S. commander in Afghanistan General John Allen has offered his condolences to families and loved ones of any members of the Pakistani security forces who may have died or were wounded.
Prime Minister Yousef Raza Gilani has ordered the Foreign Ministry to take up the matter in "the strongest terms" with NATO and the United States.
Information Minister Firdous Ashiq Awan told reporters that Pakistan "cannot tolerate such actions."
U.S. officials have said Pakistan's tribal belt provides sanctuary to the Taliban, which has been fighting for 10 years against U.S. troops in Afghanistan.
The helicopter raid comes a little more than a year after a similar, less deadly operation in which U.S. helicopters killed two Pakistani soldiers mistaken for insurgents near the Afghan border. Pakistan responded to that attack by closing down one of its border crossings to NATO supplies for more than a week until the United States apologized.
Ties between Washington and Islamabad have been unraveling since a covert U.S. commando raid in May killed al-Qaida leader Osama bin Laden who was hiding for years in a Pakistani garrison town. Pakistan was outraged it was not informed beforehand and angered by what it saw as a U.S. violation of its sovereignty.