A heavy bombardment of Tripoli also began after nightfall, with at least nine loud explosions and anti-aircraft fire heard, an Associated Press reporter in the city said.
Earlier in the day, rebels regained two key oil complexes along the coastal highway that runs from the opposition-held eastern half of the country toward Sirte and beyond that, to the capital. Moving quickly westward, the advance retraced their steps in the first rebel march toward the capital. But this time, the world's most powerful air forces have eased the way by pounding Gadhafi's military assets for the past week.
Sirte is strategically located about halfway between the rebel-held east and the Gadhafi-controlled west along the Mediterranean coast. It is a bastion of support for Gadhafi that will be difficult for the rebels to overrun and the entrances to the city have reportedly been mined. If the rebels could somehow overcome Sirte, momentum for a march on the capital would skyrocket.
An AP reporter at the front said the latest rebel advance during the day reached as far west as the oil port of Ras Lanouf, about 130 miles (210 kilometers) east of Sirte.
After nightfall, Libyan state television confirmed air raids on Sirte and Tripoli. Foreign journalists who were taken by the regime to Sirte a few hours before the bombings began reported hearing at least six loud explosions and warplanes flying overheard. They were driven around the city and said it was swarming with soldiers on patrol and armed civilians, many of them wearing green bandanas that signaled their support for Gadhafi.
In the contested city of Misrata in western Libya, residents reported fighting between rebels and Gadhafi loyalists who fired from tanks on residential areas. Misrata is one of two cities in western Libya that have risen against the regime and suffered brutal crackdowns. It is located between Tripoli and Sirte on the coastal road.
In Washington, Defense Secretary Robert Gates said he could not offer a timetable for how long the Libya operation could last, as the Obama administration tried to bolster its case for bringing the United States into another war in the Muslim world.
The U.N. Security Council authorized the operation to protect Libyan civilians after Gadhafi launched attacks against anti-government protesters who demanded that he step down after nearly 42 years in power. The airstrikes have crippled Gadhafi's forces, allowing rebels to advance less than two weeks after they had seemed at the brink of defeat.
Now that the rebels have regained control of two key oil ports, they are making tentative plans to exploit Libya's most valuable natural resource. But production is at a trickle, the foreign oil workers and their vital expertise have fled the country, and even talk of a marketing deal with Qatar seems murky at best.
"As they move round the coast, of course, the rebels will increasingly control the exit points of Libya's oil," British Defense Secretary Liam Fox told the BBC. "That will produce a very dynamic and a very different equilibrium inside Libya. How that will play out in terms of public opinion and the Gadhafi regime remains to be seen."
The coastal complexes at Ras Lanouf and Brega were responsible for a large chunk of Libya's 1.5 million barrels of daily exports, which have all but stopped since the uprising that began Feb. 15 and was inspired by the toppling of governments in Tunisia and Egypt.
On the eastern approach of Ras Lanouf, airstrikes hit three empty tank transporters and left two buildings that appeared to be sleeping quarters pockmarked with shrapnel. Like the oil port of Brega and the city of Ajdabiya before it, Gadhafi's troops appear to have left in a hurry, abandoning ammunition and disappearing without a fight.
"There was no resistance. Gadhafi's forces just melted away," said Suleiman Ibrahim, a 31-year-old volunteer, sitting in the back of a pickup truck on the road between the two towns. "This couldn't have happened without NATO. They gave us big support."
The agreement with the tiny Gulf nation of Qatar could allow the rebels to exploit Libya's vast oil reserves -- most of which are in the eastern territory they control. With no ships coming or going, Libya's tanks are full to the brim. Until they are emptied, there's nowhere to store any oil that is pumped from the ground.
Qatar, which has conducted at least one sortie over Libya, is the only Arab country known to have actively joined with the international force.
"We trust them, so basically they are the ones who are going to market our oil for us," Ali Tarhouni, the rebel finance official, told The Associated Press on Friday. "For Qatar there's no words to describe what they've done for the Libyan cause."
Officials at Qatar's ministry of energy and industry could not be reached for comment. Executives with the Arabian Gulf Oil Co., the National Oil Co. subsidiary in the east that broke free from its parent company, also could not be reached. Repeated calls to Libya's oil minister went unanswered.
Eastern oil officials said over a week ago they were still producing about 100,000 barrels per day from two key fields. But it was unclear whether such levels were sustainable given the security problems across the country and the exodus of foreign workers from the vital sector.
The Paris-based International Energy Agency said recently it believed that Libyan oil production had "slowed to a trickle" while exports had "ground to a halt." The IEA said it believed it could take months for Libyan oil to reappear on the world market.
Gates said the international action appeared to be a success, with the no-fly zone in place and sustainable with "a lot less effort than it took to set it up." He said the Pentagon was planning how to draw down resources that will be assigned to European and other countries pledging to take on a larger role.
But asked on ABC's "This Week" if that would mean a U.S. military commitment until year's end, Gates said, "I don't think anybody knows the answer to that."
The Gadhafi regime on Saturday acknowledged the airstrikes had forced its troops to retreat and accused international forces of choosing sides.
"This is the objective of the coalition now, it is not to protect civilians because now they are directly fighting against the armed forces," Khaled Kaim, the deputy foreign minister, said in the capital, Tripoli. "They are trying to push the country to the brink of a civil war."
The rebel turnaround is a boost for President Barack Obama, who has faced complaints from lawmakers from both parties that he has not sought their input about the U.S. role in the conflict or explained with enough clarity about the American goals and exit strategy.
Obama was expected to give a speech to the nation Monday, and U.S. Secretary of State Hillary Rodham Clinton on Sunday defended the administration's decision.
Libya, she told CBS, "had a leader who used military force against the protesters from one end of his country to the other, who publicly said things like "we'll show no mercy," "we'll go house to house," and the international community moved with great speed in part because there's a history here."
Pentagon officials are looking at plans to expand the firepower and airborne surveillance systems in the military campaign, including using the Air Force's AC-130 gunship armed with cannons that shoot from the side doors, as well as helicopters and drones.
Fox, the British foreign minister, ruled out supplying arms to the rebels. "We are not arming the rebels, we are not planning to arm the rebels," he said.
Al-Shalchi reported from Tripoli.