Nato has refused to apologise for a "friendly fire" attack on rebel tanks in eastern Libya that killed at least four people.
Rear Adm Russ Harding said that until Thursday's incident, Nato had not been aware that rebel troops had started to use tanks.
"Our job is to protect civilians," he told a news conference.
Rebel forces reacted with anger at the air strike on their tanks near the eastern town of Ajdabiya.
However, rebel commanders had stressed that it would not damage relations with the allied force.
Rear Adm Harding, speaking in Naples, described the situation between the towns of Ajdabiya and Brega - where the attack happened - as "very fluid" with vehicles "going backwards and forwards".
He said government tanks known to have previously targeted civilians in the town of Misrata had been on the road on Thursday. At that point, Nato did not know that rebel troops had begun to bring out their tanks.
He said that the UN mandate under which Nato is operating is to "protect civilians", whether from the forces of Col Gaddafi or the rebel troops themselves.
"Let me be clear, it did not say to gain the trust of either side in doing that," he said.
The rebels hit in Thursday's air strike had been moving a group of tanks, armoured vehicles and rocket launchers near the front line between the towns of Ajdabiya and Brega in more than 30 transporters.
One rebel commander told the BBC he saw at least four missiles land among rebel fighters.
Rebels said four rebels died, while local doctors told the BBC at least 13 fighters had been killed in the strike. Many more were injured.
The BBC's Wyre Davies in Ajdabiya said there was considerable anger among rebel troops about the incident. They were asking why rebel units were hit, when they could be seen clearly advancing in a westerly direction towards the front line.
Rebel commander Gen Abdelfatah Yunis had earlier called on Nato to give a "rational and convincing explanation" about the incident.
He also said such mistakes must not be repeated and called for better co-operation in the future.