BRUSSELS (AP) -- NATO acknowledged Friday that its airstrikes had hit rebels using tanks to fight government forces in eastern Libya, saying no one told them the rebels used tanks.

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British Rear Adm. Russell Harding, the deputy commander of the NATO operation, said in the past, only forces loyal to Libyan strongman Moammar Gadhafi had used heavy armored vehicles.

Harding says the rebels and government troops are engaged in a series of advances and retreats between the eastern coastal towns of Brega and Ajdabiya, making it difficult for pilots to distinguish between them.

NATO jets attacked a rebel convoy between these two towns Thursday, killing at least five fighters and destroying or damaging a number of armored vehicles.

"It would appear that two of our strikes yesterday may have resulted in (rebel) deaths," Harding told reporters in Naples, where the alliance's operational center is located.

"I am not apologizing," he said. "The situation on the ground was and remains extremely fluid, and until yesterday we did not have information that (rebel) forces are using tanks."

The strikes, including an attack earlier this week, provoked angry denunciations of NATO by the rebels. At the same time, NATO officials have expressed frustration with the Libyan insurgents, who now view the alliance, whose mandate is limited to protecting civilians in Libya, as their proxy air force.

NATO's Secretary-General Anders Fogh Rasmussen, however, expressed regret over the loss of life, saying alliance forces were doing everything possible to avoid harming civilians.

NATO last week took control over the international airstrikes that began March 19 as a U.S.-led mission. The airstrikes thwarted Gadhafi's efforts to crush the rebellion in the North African nation he has ruled for more than four decades, but the rebels remain outnumbered and outgunned and have had difficulty pushing into government-held territory even with air support.

Harding said Friday that NATO jets had conducted 318 sorties and struck 23 targets across Libya in the past 48 hours. They have flown over 1,500 sorties in the eight days since the alliance assumed overall command from a U.S.-led force.

NATO's jets have destroyed Gadhafi's anti-aircraft missile defenses, T-72 tanks and ammunition dumps, Harding said. The attacks also targeted Gadhafi's loyalist forces in the besieged city of Misrata, where rebels continue to hold out.

Critics have questioned NATO's limited strategy of only protecting civilians threatened by Gadhafi's troops, rather than trying to eliminate the threat completely by destroying the strongman's regime.

"By not striking at the regime from the outset, Gadhafi was granted the initiative to embed his forces in urban settings hiding behind human shields in a form of guerrilla warfare," said Barack Seneer, a researcher on the Middle East at the Royal United Services Institute, a British military think tank.

"A no-fly zone is not equipped to contend with guerrilla warfare or with a stalemate that places rebels and loyalists at close proximity with one another." he said

Despite the attacks on anti-aircraft sites, Gadhafi's forces still pose a danger for NATO warplanes. They retain radars and surface-to-air missiles, as well as automatic cannons and shoulder-launched missiles that can hit planes at altitudes up to 5,000 meters (15,000 feet).

Over the past week, Gadhafi's forces had switched tactics by leaving their heavy armor behind and using only light trucks armed with heavy machine guns and fast-firing anti-aircraft cannons on the front lines between Brega and Ajdabiya. These have proven very effective in disrupting repeated rebel attempts to push west toward Tripoli, but Gadhafi's forces have not been able to drive the rebels back toward Benghazi or establish a solid front line in that sector.

"These trucks cannot hold ground," Harding said. "When you see their tanks coming up, those are the vehicles that can cause the greatest harm to civilians."

On Thursday, the situation in that sector "was very confusing, vehicles going back and forth," he said.