A Nato air strike intended to thwart Muammar Gaddafi's forces killed 13 rebel fighters in eastern Libya, the opposition said.
But they described yesterday's incident as an "unfortunate accident" and stressed it did not diminish their support for the international air campaign.
The rebels' response to the attack - blaming it on a mistake within their ranks - highlighted their heavy dependence on the international air campaign as they face the superior military power of the long-time Libyan leader. The misfire also showed the challenges the coalition faces in identifying targets without co-ordination with forces on the ground.
"As regrettable as it may be, we understand that we might have to give up lives for the greater good. We have to look at the bigger picture," opposition spokesman Mustafa Gheriani said. "This is a war and the lines are so fluid going back and forth, so it's natural that mistakes will happen."
The fighters were hit on Friday night as they moved forward, attempting to take back the oil city of Brega while air strikes were in progress. Seven fighters were injured.
Another opposition spokesman, Abdel-Hafidh Ghoga, said it was an example of the lack of co-ordination in the ranks that has proved a key obstacle to victory over the more organised Libyan military.
Rebels without training - sometimes even without weapons - have rushed in and out of fighting in a free-for-all for more than six weeks, repeatedly getting trounced by Gaddafi's more heavily-armed forces. But ex-military officers who have joined the rebel side have stepped up training efforts and taken a greater role in the fight.
Sorting rebels from Gaddafi's forces has become more difficult recently, as some loyalists have given up tanks and other armoured vehicles for the kind of equipment the rebels rely on - pick-up trucks and other vehicles equipped with makeshift armaments.
Nato spokeswoman Oana Lungescu said the alliance was investigating the reports. "The exact details are hard to verify because we have no reliable source on the ground," Ms Lungescu said. "Clearly, if someone fires at one of our aircraft they have the right to defend themselves."
Nato, which on Thursday took over what had been a US-led military campaign to stop Gaddafi from attacking his own people, also is investigating whether other air strikes have killed civilians in western Libya, as the Libyan government claims.