Rebel fighters fled under fire from a key town in eastern Libya on Tuesday as world leaders convening in London insisted that Moammar Gaddafi step down but offered no new suggestions for how to dislodge him from power.

The rebels’ chaotic retreat from the town of Bin Jawwad, which they had captured from troops loyal to Gaddafi just two days earlier, reversed the momentum they had seized over the weekend and suggested that the ad hoc and lightly armed opposition force may have reached the limits of its capacity.

It was the fourth time Bin Jawwad has changed hands in less than three weeks, raising the specter of a prolonged stalemate along the sparsely populated stretch of coastal highway between the rebel stronghold of Benghazi to the east and Gaddafi’s heavily garrisoned home town of Sirte to the west.

Although the 40 world leaders meeting in London pledged humanitarian aid and continued airstrikes to protect civilians, they indicated that it would be up to the Libyans themselves to force Gaddafi out, leaving it unclear how they were supposed to do so.

The question of whether to arm the rebels was not publicly discussed, nor was the question of how to release frozen Libyan assets to help fund them. But the leaders attending the conference made it clear that the military campaign in Libya would not end until Gaddafi had gone.

“Gaddafi has lost the legitimacy to lead, so we believe he must go. We’re working with the international community to try to achieve that outcome,” U.S. Secretary of State Hillary Rodham Clinton told reporters after the talks, indicating that the United States is pinning its hopes on defections from those around Gaddafi.

President Obama said Tuesday that he would not preclude the possibility of arming the rebels. Pressed on the issue in an interview with NBC News, Obama said, “I’m not ruling it out, but I’m also not ruling it in.”

“We are still making an assessment about what Gaddafi’s forces are doing,” the president said.

In a series of interviews with the three major television networks, Obama emphasized that his decision to deploy U.S. forces in Libya should not be applied to other countries in the Middle East and North Africa. He told NBC that his policy on Libya should not be construed as an “Obama doctrine” that can be applied in a “cookie-cutter fashion.”

The strongest challenge to Gaddafi in London came from the prime minister and foreign minister of Qatar, a nation that has been the most forthright Arab supporter of the Western-led military campaign in Libya. Hamad Bin Jasim al-Thani, the prime minister, warned “Gaddafi and his people to leave and not cause any more bloodshed.”

“Right now, we don’t see any indication of that,” he said. “But this hope which we offer now might not be on the table after a few days. I am not warning anybody here, but I’m trying to stop the bloodshed as soon as possible.”

Clinton and other leaders reiterated their conviction that the military campaign in Libya has saved lives by reversing the advance of Gaddafi forces toward Benghazi.

“We have prevented a potential massacre, established a no-fly zone, stopped an advancing army, added more partners to this coalition and transferred command of the military effort to NATO,” Clinton said. “That’s not bad for a week of work at a time of great, intense international concern.”

Gaddafi has not been seen or heard from publicly in a week, but with his forces advancing east on the 11th day of airstrikes, no immediate pressure appeared on his government to abandon him.

Bin Jawwad, 90 miles east of Sirte, marked the farthest point of the rebel advance the last time they swept west through government lines a little over three weeks ago. The retreat Tuesday suggested that the rebels will have a difficult time taking and holding territory in Gaddafi’s loyalist heartland.

News footage showed images of panicked rebels leaping into cars and pickup trucks and scrambling to leave Bin Jawwad as approaching Gaddafi forces pounded them with mortar shells and artillery fire. There were no reports of coalition airstrikes as the rebels withdrew.

The rebels retreated 37 miles east to Ras Lanuf, the oil refinery town they had retaken from Gaddafi earlier this month as the momentum in the war seemed to swing in their favor. Yet even there, their hold seemed tenuous. Reports late Tuesday said the town was coming under heavy artillery fire from advancing Gaddafi troops.

There were also reports from the besieged town of Misurata that Libyan forces had launched a fresh onslaught of attacks, pounding civilian areas with mortar and artillery fire. Four brothers were killed, according to a physician at a rebel-controlled hospital. In Tripoli, airstrikes occurred for the first time in daylight, with three loud explosions shaking the capital at 5:30 p.m.

As news of the rebels’ retreat reached Benghazi, the mood was somber. Rebel spokeswoman Iman Bugaighis described the action as a “tactical withdrawal” designed to take rebel forces “out of the range of Col. Gaddafi’s militia and mercenary troops.”

Rebel officials nonetheless said they welcomed the London conference for the increased diplomatic recognition it appeared to afford their self-styled government, the Transitional National Council.

“We don’t have arms,” said Guma El-Gamaty, British co-coordinator for the council, who added that he would welcome offers to provide weapons to the rebels. “But we ask for political support more than we ask for arms.”

Clinton, like Obama, did not discount the possibility of arming the rebels. She said she thought such a step would be legal under the terms of U.N. Security Council Resolution 1973, which authorized the use of force to protect the lives of Libyan civilians. But British Foreign Secretary William Hague said the subject had not been raised.

U.S. and European leaders met with leaders of the rebel council and set up a multinational Libyan contact group to coordinate political strategy in the weeks ahead.

Warrick reported from London. Staff writers Tara Bahrampour in Benghazi, Libya, and Perry Bacon Jr. in Washington contributed to this report.