— An American fighter jet participating in the U.S.-led airstrikes in Libya malfunctioned on Monday, and both its crew members ejected safely before it crashed on Libyan soil, the U.S. military said.

The U.S. Africa Command, which is leading the international military campaign in Libya, said in a statement that the two crew members “are safe.”

News service and television reports said the men encountered rebel fighters when they crashed, and were treated well by them. Both men were subsequently evacuated from Libya by the U.S.-led military coalition, according to those news reports.

The military statements said the F-15E jet “experienced equipment malfunction over northeast Libya.” The cause of the malfunction, the statement said, is under investigation.

Outside this strategic eastern city on Monday, which is partly controlled by forces loyal to Libyan leader Moammar Gaddafi, opposition fighters cheered and flashed victory signs as allied jets streaked overhead.

But as the rebels sped toward Ajdabiya in dozens of vehicles, many mounted with machine guns, Gaddafi’s soldiers attacked. They rained mortar and tank shells that exploded like thunderclaps, spraying thick smoke and debris along the highway. The rebels quickly retreated.

Even as allied strikes hammer Gaddafi’s air defenses, his ground forces have dug in within heavily populated urban areas such as Ajdabiya, and on Monday they gained ground in the western city of Misurata.

U.S. officials say the three-day-old international military intervention is intended to protect Libyan civilians, not provide support to Libya’s opposition. But Monday’s setbacks for the rebels revealed the degree to which the disorganized and ill-equipped force is depending on allied airstrikes to end Gaddafi’s 41-year rule. It also raised questions, so far unresolved, about how far coalition members are prepared to go to help Libya’s opposition.

On Monday, NATO members voiced disagreement over the goal and leadership of the international mission in Libya. The United States had hoped to turn command of the operation over to NATO, but that transition appears to have been delayed by the lack of consensus within the organization.

U.S. officials maintained Monday that they were interested only in shielding civilians from violence and that getting rid of Gaddafi would be up to the Lib­yan people. But rebels said more assistance was needed to avert the massacre that would inevitably come if Gaddafi was allowed to remain in power.

“We can’t win without the airplanes of the international community,” Farhad al-Mraibi, a 55-year-old rebel fighter, said after the retreat. “Gaddafi will kill all of us.”

Top rebel officials say the internationally enforced no-fly zone has come too late to alter the military equation on the ground. Their forces, they say, are not militarily equipped to battle Gaddafi’s superior arsenal of tanks, rocket launchers and other heavy land-based weaponry.

Preventing Gaddafi from laying siege to cities falls under the U.N. mandate of protecting civilians, and rebel leaders said Monday that more allied airstrikes are needed to swiftly immobilize his ground forces. Although they rejected the idea of foreign troops on Libyan soil, they said the international intervention must be about regime change — a position that U.S. officials have pointedly declined to take.

“I believe this is about pushing Gaddafi out of power,” said Salwa el-Daghili, a constitutional law professor and member of the rebel provisional leadership. “They must quickly attack Gaddafi’s ground forces.”

Since the U.N. Security Council vote authorizing the use of force last week, Gaddafi’s ground forces have attempted to retake the rebel stronghold of Benghazi, an offensive that was repelled only after allied airstrikes decimated Gaddafi’s encampments outside the city.

Rebels are concerned, however, that the coalition has focused on Benghazi to the exclusion of other areas where rebels are holding out against government forces, notably Misurata, Libya’s third-largest city. There, Gaddafi’s forces launched a major assault on the eve of the U.N. vote.

On Monday, loyalist forces reportedly made further advances, with government tanks rolling into the city and attempting to fight their way to the symbolic central square. At least 14 people were killed and more than 100 were injured, according to a doctor at the hospital in Misurata who spoke on the condition of anonymity because he feared for his security.

He said that rebel fighters managed to hold on to the square but that Gaddafi’s forces had secured control of several neighborhoods and positioned snipers on buildings on a main road leading through the city. Tanks and artillery continued to pound rebel-held neighborhoods throughout the day, he said. Overnight, a helicopter attacked the antenna of the local radio station.

“What no-fly zone?” the doctor asked. “I am sure the U.N. has forgotten us.”

Government spokesman Moussa Ibrahim said that Misurata had been “liberated” but that pockets of what he described as Islamist extremists were continuing to resist government forces.

He said coalition forces had struck a harbor west of Tripoli on Monday and the airports in Gaddafi’s hometown of Sirte and in the southern town of Sabha in earlier strikes. He said that all the targets were civilian facilities and that “many” civilians had been killed, an assertion that could not be independently verified.

The fight for Ajdabiya

Pushing Gaddafi’s soldiers out of Ajdabiya, a city of 170,000 people that is roughly 100 miles south of Benghazi, would bring a tactical and psychological victory to the rebels. Nestled on a strategic highway junction, the city would build an additional layer of protection for Benghazi, the birthplace of Libya’s populist rebellion. It would also solidify the rebel movement’s grip over eastern Libya.

For the rebel forces, Monday began with an abundance of hope and confidence. A day after the allied airstrikes, the highway from Benghazi to Ajdabiya was a graveyard for Gaddafi’s forces. Charred tanks, some still burning, and shattered armored personnel carriers littered the road beside the bodies of Gaddafi’s soldiers.

But near Ajdabiya there was anxiety brewing. Rebel commanders said they were unable to reach their allies inside the city to mount a two-pronged offensive against Gaddafi’s forces positioned at the eastern entrance. Cellphones had been shut down, and the rebels had no satellite phones.

“Why doesn’t our leadership give us better communications?” said Mraibi, the rebel fighter. “We are cut off from our forces inside Ajdabiya.”

Like other fighters, he said he was expecting an allied airstrike on Gaddafi’s tanks and rocket launchers on the perimeter of the city. Adil al-Hasi, a rebel commander, said he was given orders not to send his men into battle until allied jets had struck Gaddafi’s forces.

“Gaddafi’s forces are like birds,” said Fathi Bin Saoud, 54, a fighter. “They can fly anywhere, but the international community can shoot them down. Our role in this war is to walk in and pluck their feathers.”

So when the rebels saw the coalition jets, there was a sense of relief and euphoria. Fighters pointed at the sky and cheered. “The French are here,” yelled one, referring to French warplanes leading the allied air assault.

After the retreat, rebel forces were in disarray. Within minutes, the front line had been pushed back five miles. “We made a mistake,” mumbled Kareem Ali, 55, another fighter, as he looked toward Ajdabiya.

Several rebels had been killed, including four in a pickup truck that was covered with blood. Rebel commanders tried to regroup their men, but they failed to bring order. Some fighters proposed taking a back route through the desert to attack Gaddafi’s forces. Others inexplicably began to fire heavy machine guns into the sky, which raised concerns that the allied jets might strike them instead.

“Every one of us has his own plan,” a fighter named Abu Ahmed, clutching a ragged pair of binoculars, shouted at a comrade.

In the end, though, almost every commander and fighter reached the same conclusion: Don’t push forward. Let the allied warplanes destroy Gaddafi’s forces. Some hoped that their foes, fearing the airstrikes, would leave on their own.

“We are waiting,” said Hasi, the commander. “Maybe they will surrender.”

Correspondent Liz Sly in Tripoli and staff writers Greg Jaffe and Debbi Wilgoren in Washington contributed to this report.