NATO and Afghan forces came under fire from across the border with Pakistan before NATO aircraft attacked a Pakistani army post, killing 24 soldiers, a Western official and a senior Afghan security official said on Sunday.
Pakistan has said the attack was an unprovoked assault. The NATO-led coalition in Afghanistan has made little comment, apart from offering condolences for the deaths and admitting it was "highly likely" they had been caused by its forces.
"They came under cross-border fire," the Western official said.
The Afghan official said troops had come under fire from inside Pakistan as they were descending from helicopters, which had returned fire.
Both officials asked not to be named because the attack is so sensitive. It has severely strained already fractious relations between the United States and Islamabad.
