TRIPOLI, Libya – Rebels surged westward along Libya’s coast Sunday, seizing three more key towns and capitalizing on their new momentum after more than a week of airstrikes by an international coalition.

U.S. officials were cautiously optimistic about the reversal of fortunes for the rebels. President Obama is scheduled to address the nation at 7:30 p.m. today, and officials said he will be able to show that the operation is starting to achieve its goals. Obama has faced mounting criticism from some lawmakers, who fear that the United States could get bogged down in a foreign intervention without a clear objective.

Although the rebels have seized the initiative in eastern Libya, they still face formidable obstacles, analysts warned. Senior U.S. officials said Sunday that Moammar Gadhafi’s 42-year rule could end with the implosion of his regime or a negotiated settlement rather than an outright rebel victory.

“One should not underestimate the possibility of the regime itself cracking,” Defense Secretary Robert Gates said Sunday on NBC’s “Meet the Press.”

NATO members agreed Sunday evening that the alliance would assume control of the international military campaign against Libya. It had earlier taken over from the U.S. military in leading enforcement of an arms embargo and a no-fly zone and had debated for days whether to coordinate the politically riskier strikes on Libyan ground forces.

With Gadhafi’s air-defense equipment largely destroyed and NATO stepping up to assume command, the United States will be able to reduce its role, Gates said.

“Within the next week or so, we will begin to diminish the commitment of resources,” Gates said. He added, however, that the United States would stay on in a supporting role, and acknowledged that it was unclear how long the operation would last. He and Secretary of State Hillary Rodham Clinton on Sunday defended the Libya campaign, saying it was helping avert a humanitarian catastrophe.

The coalition again targeted the capital, Tripoli, with witnesses reporting at least 10 loud explosions Sunday night, followed by bursts of antiaircraft fire.

As rebel forces headed toward Sirte, 278 miles east of Tripoli, reporters on a government trip to the city heard at least half a dozen explosions and saw warplanes circling. The heavily guarded city, Gadhafi’s hometown, is expected to pose the toughest challenge yet to the rebels.

Libyan state television reported what it said were the first coalition strikes against Sirte, but by late Sunday it had still not broadcast details of the loyalist army’s rapid retreat across more than 200 miles of coastal highway over the previous 24 hours.

Government spokesman Moussa Ibrahim said Sunday evening that although its forces had pulled back, “we are still very strong on the ground.” He said the airstrikes were “a plan to (put) the Libyan state in a weak negotiating position.”

The rebels’ recovery began Friday, when they recaptured the strategic town of Ajdabiya after coalition bombings weakened Gadhafi’s forces. The rebels then charged on to Brega and Ras Lanuf, sparsely populated petroleum centers, and took over coastal Bin Jawwad. They had been repulsed at Bin Jawwad earlier in the month, as Gadhafi’s forces launched an aggressive counterattack.

“The big question now is, can they really go beyond where they were three weeks ago?” said Dirk Vandewalle, a Libya specialist at Dartmouth College, referring to the rebels.

The coalition has focused its attacks on Libyan soldiers, tanks and weaponry outside the cities. But it has hesitated to strike inside urban areas for fear of killing civilians. Gadhafi, however, has soldiers and armored vehicles inside cities such as Misurata, a rebel outpost in western Libya where fighting continued Sunday.

The latest rebel victory in eastern Libya “doesn’t necessarily mean that Gadhafi’s days are numbered,” said Andrew Krepinevich, director of the Center for Strategic and Budgetary Assessments.

Obama has urged Gadhafi to relinquish power. But Gates said Sunday that the conflict “eventually is going to have to be settled by the Libyans themselves. Perhaps the U.N. can mediate. … But in terms of commitment, the president has put some very strict limitations in terms of what we are prepared to do.”