However, the rebel forces described it 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 longtime Libyan leader. The misfire also showed the challenges the coalition faces in identifying targets without coordination 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 slain fighters were hit Friday night as they moved forward, attempting to take back the oil city of Brega, while airstrikes were in progress. Seven fighters were injured. Another opposition spokesman, Abdel-Hafidh Ghoga, said it was an example of the lack of coordination in the ranks that has proved a key obstacle to victory over the more organized 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 Gadhafi'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.
"This unfortunate accident was a mistake that was caused by the rebels' advance during the coalition's attack," Ghoga said. "Now the military leadership that has been organized more effectively recently is working on preventing the recurrence of these accidents."
Sorting rebels from Gadhafi's forces has become more difficult recently, as some loyalists have given up tanks and other armored vehicles for the kind of equipment the rebels rely on: pickup trucks and other vehicles equipped with makeshift armaments.
NATO spokeswoman Oana Lungescu said the alliance was investigating the reports.