The United States military has expressed deep regret and condolences for a NATO airstrike in November which killed 24 Pakistan soldiers. It says a number of mistakes were made.
A statement said US and Afghan troops acted in self defence, but conceded there had been a lack of proper co-ordination with Pakistani forces in Mohmand tribal agency.
In retaliation for the killings on 26 November, Pakistan has closed its border with Afghanistan, cutting NATO supply lines.
The statement said incorrect mapping information was to blame.
The BBC reports there was no immediate response from Pakistan.
Brigadier General Stephen Clark said US and Afghan forces had landed in the area to search a village and had been fired upon from a ridge above it.
He said the coalition forces staged a show of force to make it clear who they were. But when firing continued from the ridge, air strikes were carried out.
Troops searched the village that the next day and uncovered ''a considerable cache'' of weapons.