Tripoli, Libya (CNN) -- NATO's top official "strongly regretted" a fatal airstrike that may have mistakenly killed people it has pledged to protect, angering Libyan opposition leaders amid an increasingly frustrating campaign to oust Moammar Gadhafi.
NATO Secretary-General Anders Fogh Rasmussen called the incident unfortunate and said he regretted the loss of life.
"We are conducting operations in Libya in accordance with the U.N. Security Council resolution with the aim to protect civilians," he said. "This is also the reason why our aircraft target military equipment that could be used to attack civilians, but I can assure you that we do our utmost to avoid civilian casualties."
His comments came after the the deputy commander of the NATO operations refused to apologize
Gadhafi's forces attacked the western city of Ajdabiya on Friday with a barrage of artillery fire at the city's western gates. Ajdabiya has changed hands several times already, and rebels were forced to flee again in a war that is now viewed in some circles as unwinnable for the opposition, even with NATO air support.
NATO, meanwhile, was on the defensive Friday after reports of casualties apparently caused by the airstrike. British Royal Navy Rear Adm. Russell Harding said NATO forces may have hit rebel tanks near the eastern oil town of al-Brega on Thursday.