NATO helicopters and fighter jets attacked two Pakistan military outposts on Saturday, killing 24 soldiers in what Pakistan said was an unprovoked assault.
The incident has provoked a new crisis in the fraught relations between the United States, which leads the NATO mission in Afghanistan, and its ally Pakistan, where anti-U.S. public sentiment is high.
NATO Secretary-General Anders Fogh Rasmussen called the incident "regrettable" and said he had written to Prime Minister Yousaf Raza Gilani to say that the deaths of Pakistani personnel were "as unacceptable and deplorable as the deaths of Afghan and international personnel."
"This was a tragic unintended incident," Rasmussen said in a statement, adding that he fully supported a NATO investigation that was underway. "We will determine what happened, and draw the right lessons."
Rasmussen stressed the joint interest of NATO and Pakistan in the fight against Islamist militants and said the alliance was committed to working with Pakistan to ensure such incidents did not recur.
While both NATO and US officials expressed regret about the deaths of the soldiers, the exact circumstances of the attacks remain unclear.
Pakistan has shut down NATO supply routes into Afghanistan - used for sending in nearly half of the alliance's land shipments - in retaliation. A similar incident on Sept 30, 2010, which killed two Pakistani service personnel, led to the closure of one of NATO's supply routes through Pakistan for 10 days.