NATO grudgingly expressed regret Friday for mistakenly bombing rebel forces in Libya and killing at least five people the previous day, as the limitations of the alliance’s mandate, the effectiveness of its operations and its fraying relationship with the rebels came under further scrutiny.

The deputy commander of NATO operations in Libya, Rear Adm. Russell Harding, first sought to shift the blame for Thursday’s bombing to rebel commanders, who he said had deployed captured Libyan army tanks for the first time, unbeknownst to NATO pilots.

“I’m not apologizing,” Harding said in remarks from his headquarters in Naples. “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.

On the battlefront, heavy clashes between government and opposition forces were reported Friday in Misurata, the only major rebel-held city in western Libya, as international relief efforts were stepped up for civilians caught in the crossfire. Farther east, rebels said they prevented Libyan leader Moammar Gaddafi’s forces from seizing control of the eastern city of Ajdabiya after chaotic fighting and a partial retreat Thursday. They said they had full control of the town, after reinforcements arrived from their stronghold of Benghazi.

But with a military stalemate looking ever more likely, the United States and its allies are searching for a way to break the deadlock.

“Everybody’s asking the question: What will happen next? What’s the political strategy? They’re asking in Washington, and they’re asking in Brussels,” said a U.S. official who spoke on the condition of anonymity because of the sensitivity of the issue.

The official said the attitude among the coalition is that “what we’re doing is the right thing to do.” But the official conceded that “it will take some more time. How much more, frankly, is a question we’ll face when we need to face it, which isn’t today.”

One possible way to change the dynamic is through diplomacy. But so far, that has been a dead end. Gaddafi has launched an intense diplomatic effort across Europe and beyond this week to determine whether any combination of concessions short of his resignation and departure would satisfy the demands of the international community, Obama administration officials said.

The response, at every stop, has been no. But officials have interpreted the outreach as a sign of weakening in Gaddafi’s belief that gains by his military forces on the ground can ultimately quash a rebel army and outlast international determination and solidarity against him.

Gaddafi emissaries have traveled to Turkey, Italy, Norway, Spain, Greece, Malta and elsewhere with offers of a cease-fire, a rollback of government troops and a transitional government leading to a democratic political process, according to officials who agreed to discuss the sensitive Libya situation on condition they not be quoted or identified.

Libya’s opposition Transitional National Council and the Obama administration have said that Gaddafi’s departure is a non-negotiable demand. They have also demanded the departure of Gaddafi’s sons, which some members of the coalition feel less strongly about.

‘Emotions can flare up’

Burying their dead in Benghazi, rebels were angry over Thursday’s loss of life by friendly fire and promised to continue the fight against Gaddafi. But their leaders on Friday tried to play down the tensions with NATO.

“We have fluid conditions and emotions can flare up,” said Mustafa Gheriani, a spokesman for the opposition. “In the end you have to decide, does the benefit outweigh the negative?’’ We have to forgive the mistake because the benefit of them protecting civilians way outweighs the pitfalls.”

Nevertheless, NATO’s 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 colors of the rebel flag.

But Harding blamed the rebels for the mistake. The NATO mission, as defined by the U.N. Security Council, is to protect Libyan civilians from attack by forces loyal to Gaddafi, and not to serve as the air wing of rebel forces, Harding 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 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.

Despite Harding’s insistence that protecting civilians was NATO’s only mission, leaders of NATO countries have repeatedly called for Gaddafi to step down. Rebel commanders have said they are relying on Western airstrikes to destroy Libyan forces involved in the back-and-forth battles raging along the coastal highway.

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.”

Fadel reported from Benghazi. Staff writers Karen DeYoung in Washington and Craig Timberg in Mosul, Iraq, and correspondent Simon Denyer in Tripoli contributed to this report.