Pakistan said the /*NATO*/ aircraft fired on two army checkpoints, killing 26 soldiers in northwestern /*Pakistan*/ early Saturday. Its army chief calls it a "blatant and unacceptable act."
A NATO spokesman said it's "highly likely" that close air support called in by Afghan and coalition forces operating in the border area caused Pakistani casualties. He told BBC television that NATO is trying to determine the exact details.
Gen. John Allen, the top overall commander of U.S. and NATO forces in Afghanistan, issued a statement expressing "heartfelt condolences" to the loved ones of those killed or injured.
In the meantime, Pakistan has closed the border-crossing used by coalition forces to supply troops in Afghanistan.
Last year, Pakistan closed one of the crossings for 10 days after U.S. helicopters accidentally killed two Pakistani who were mistaken for insurgents.
The Associated Press contributed to this report.