The deputy commander of NATO operations in Libya acknowledged Friday that NATO warplanes may have mistakenly bombed rebel forces Thursday near Brega, killing at least five people and generating angry complaints from rebel leaders.

After initially declining to apologize, NATO later voiced regret for the lethal mistake.

Rear Adm. Russell Harding, in a briefing from his Naples headquarters, sought to shift the blame for the bombing to rebel commanders, who he said had deployed captured Libyan army tanks for the first time, unbeknown to NATO pilots flying bombing raids high over the area.

“I’m not apologizing,” Harding said in remarks streamed over the Internet by NATO. “The situation on the ground, as I said, was very confused and remains very confused. And up to yesterday, we had no information that the [rebel] forces were using tanks.”

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

The admission of responsibility further underscored the lack of communication between opposition leadership in eastern Libya and NATO. The rebel military commander, Abdul Fattah Younis, said Thursday that the rebels had notified NATO they would be deploying tanks while also saying he was not in direct contact with NATO.

The tanks and bus were parked outside the oil town of Brega, other fighters said, and were marked with the green, black and red rebel flag.

Asked how communications between NATO and rebel forces could be improved to prevent more such friendly fire casualties, Harding said that was not NATO’s problem. The NATO mission, as defined by the U.N. Security Council, is to protect Libyan civilians from attack by forces loyal to Libyan leader Moammar Gaddafi, and not to serve as the air wing of rebel forces, he said.

“I have to be frank and say it is not for us, trying to protect civilians, to improve communications with rebel forces,” he added.

Harding’s stand suggested NATO officials were stung by complaints from rebels near Brega, an oil town in eastern Libya that has changed hands several times, that NATO aircraft were not providing the same level of air support since the United States relinquished direct command of Western military operations over Libya on March 31.

He emphasized repeatedly that the mission is to protect civilians and said the planes under his command are striking far and wide to destroy army equipment that might threaten them. More than 1,500 sorties — one flight by one plane — have been flown or are planned, he said, including 110 on the drawing boards for Friday.

The aircraft, mainly French and British, have destroyed T-72 tanks, anti-aircraft missile emplacements and ammunition dumps across the country, Harding said, severely degrading Gaddafi’s military strength and his ability to attack civilians in rebel-held areas, including the besieged city of Misurata.

“The civilian population of Libya is NATO’s primary concern,” said the alliance spokeswoman, Oana Lungescu, from NATO’s Brussels headquarters.

She explained that the air strikes have become more difficult because Gaddafi’s forces have started using civilian vehicles — just as the rebels do — and positioning tanks, artillery and personnel among the civilian populations of several cities, raising the risk of collateral casualties. “It’s very hard to see who’s who,” she said.

Despite Harding’s insistence that protecting civilians was his only mission, rebel commanders have said all along that they are relying on Western air strikes to destroy Libyan forces involved in the back-and-forth battles raging along the coastal highway. That has not been working since NATO assumed command, they complained.

One day after the command shift, NATO planes killed 13 rebel fighters when a pilot mistook celebratory rebel gunfire for hostile actions by Gaddafi loyalists.

In Thursday’s strikes, rebel fighters said, NATO planes appear to have again attacked a convoy of vehicles, including tanks, that had been positioned along the fringes of the battle for Brega. At least five people, including two doctors, were killed.

Some rebel officials in Benghazi, where the uprising has its headquarters, suggested late Thursday that the strikes may have been carried out by planes from Gaddafi’s forces, despite the no-fly zone being enforced by NATO.

But Harding did not address that possibility, saying: “It would appear that two of our strikes yesterday may have resulted in the deaths of at least two [rebel] personnel.”

U.S. Defense Secretary Robert M. Gates, on a visit to Mosul, Iraq, told reporters Friday that the number of NATO sorties remains about the same as it was when U.S. forces were in charge.

“It looks like the coalition is stepping up,” Gates said. “If you’re on the ground and you’re in trouble, any response is too slow.”

Correspondent Leila Fadel in Benghazi and Craig Timberg in Mosul, Iraq contributed to this report.