PHOTO: Fighting continues in Libya

A rebel fighter stands next a vehicle mounted with a high caliber machine gun as he waits for the convoy to move out after refueling at a staging post on the western outskirts of Ajdabiyah. (Getty Images / April 1, 2011)

Tripoli, Libya -- NATO refused to apologize Friday for a deadly airstrike it conceded may have mistakenly killed people it has pledged to protect, angering Libyan opposition leaders amid an increasingly frustrating campaign to oust Moammar Gadhafi.

Gadhafi's forces attacked Ajdabiya on Friday with a barrage of artillery fire at the city's western gates. Ajdabiya has changed hands several times already, and rebels were forced to flee again in a war that is now viewed in some circles as unwinnable for the opposition, even with NATO air support.

NATO, meanwhile, was on the defensive Friday after reports of casualties apparently caused by the airstrike. British Royal Navy Rear Adm. Russell Harding said NATO forces may have hit rebel tanks near the eastern oil town of al-Brega on Thursday.

Witnesses told CNN that two rebel fighters and two doctors were killed when missiles struck a rebel formation on the eastern Libyan battlefront.

It was the second time NATO has been blamed for civilian deaths -- last week, opposition leaders said NATO airstrikes killed 13 civilians in the al-Brega area. NATO is investigating that strike as well.

"I'm not apologizing," Harding, the deputy commander of the NATO operation, said of the latest incident. "The situation on the ground is fluid, and we had no information the opposition forces were using tanks."

Harding said NATO had only recently learned that opposition forces had tanks. In the past, it was Gadhafi's tanks that had taken aim at civilians, he said.

"There's a lot of vehicles going back and forth," he said. "It is very difficult to distinguish who is operating the vehicles."

The airstrikes also injured 14 people, and an additional six are missing, said Gen. Abdul Fattah Yunis, a commander of the rebel forces.

NATO Secretary General Anders Fogh Rasmussen called the incident unfortunate.

"I strongly regret the loss of life," he said.

"We are conducting operations in Libya in accordance with the U.N. Security Council resolution with the aim to protect civilians," he said. "This is also the reason why our aircraft target military equipment that could be used to attack civilians, but I can assure you that we do our utmost to avoid civilian casualties."

Sorrow quickly turned to anger at a hospital where the wounded were taken, complicating matters for opposition fighters, already demoralized by the superior firepower of the Libyan army.

"NATO, NATO, NATO! They shouldn't hit the revolutionaries. We're helpless," one person screamed.

Ahmed Abu Bakr, a doctor who came to Libya from Germany to volunteer, said he never thought that he would be treating the wounded from friendly fire.

"I am very unhappy," he said. "They came here to help us, not injure us."

After the aerial attack Thursday morning, Gadhafi's troops pushed back the rebels, retaking territory and moving the front line farther east, Yunis said.

He said the rebels notified NATO of their tank movement and of their presence.

"There is no tension between us and NATO; this is a war situation, and we understand that mistakes are made," Yunis said.