BENGHAZI, Libya - A NATO airstrike intended to thwart Moammar Gadhafi's forces killed 13 rebel fighters in eastern Libya instead, the opposition said Saturday, but they 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 proven 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 truck and other vehicles equipped with makeshift armaments.

Two men badly injured in the strike said it happened at about 8 p.m. Friday after somebody fired heavy weaponry into the air as a rebel convoy made its way from Ajdabiya toward Brega.

"We all turned to him and said, "Why the hell did you do that?'" said Ali Abdullah Zio, 28. He said it was a mistake, then pulled out of the convoy and drove back to Ajdabiya. Moments later there was an airstrike.

"We were just driving along and then everything was on fire," said 19-year-old Ibrahim al-Shahaibi. "It's fate. They must have thought we were Gadhafi's brigades when they hit us. We need to get rid of him."

Al-Shahaibi was covered up to his chest in a fuzzy brown blanket in the intensive care unit at Benghazi's Jalaa hospital. His right leg was amputated below the knee and his face had severe burns.

Zio, an economics student at Qar Younis University in Benghazi, also was lying in a hospital bed, with a swollen face and his head and hands wrapped in white bandages. He was unable to open his eyes. But he said he wasn't angry, and planned to return to the front as soon as possible.

"It's the coalition that protects us," he said.

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," Lungescu said. "Clearly, if someone fires at one of our aircraft they have the right to defend themselves."

Rebels told The Associated Press that the fighters were hit about 12 miles east of Brega, which has gone back and forth between rebel and government hands in recent weeks.

NATO, which on Thursday took over what had been a U.S.-led military campaign to stop Gadhafi from attacking his own people, also is investigating whether other airstrikes have killed civilians in western Libya, as the Libyan government claims. The United States, meanwhile, ended its role in combat missions Saturday, leaving that work for other nations.

Rebels control much of eastern Libya, but in the west the only major city they hold is Misrata, which has been besieged for weeks by Gadhafi forces who have cut off water, power and food supplies.

Medical officials said Saturday that government forces killed 37 civilians over the past two days in an unrelenting campaign of shelling and sniper fire and an attack that burned down the city's main stocks of flour and sugar.

"One of the snipers kept shooting at one car, and it caught fire. We couldn't save them. They were burnt to death," a doctor said. He said a Turkish ship that delivered medical supplies to Misrata left with some of the wounded.

Over the past month and a half, 243 people have been killed and some 1,000 wounded, according to the medical officials, who spoke to the AP on condition of anonymity for fear of retaliation.

The hospital official in Misrata said Gadhafi forces were trying to pummel the port, hoping to cut off the city's last line to the outside world. He said tanks positioned on the city's east fired shells toward the area. Other Gadhafi troops disguised in civilian cars sped to the area, firing mortars and then fleeing, he said.

Gadhafi's forces have shelled the city's outskirts and residents are piling into the heart of Misrata, crowding into the homes of relations and even unfinished buildings, the hospital official said. The reports could not be independently confirmed.

Ukraine said it would dispatch a military ship to Tripoli, the Libyan capital, to collect up to 600 Ukrainians, Americans, Britons, Russians, Belorussians and other foreigners, Ukrainian spokesman Aleksandr Dikusarov said. He said Libyan authorities had guaranteed the safety of the evacuation.