BIN JAWWAD, Libya — Rebel forces bore down Monday on Moammar Gadhafi's hometown of Sirte, a key government stronghold where a brigade headed by one of the Libyan leader's sons was digging in to defend the city and setting the stage for a bloody and possibly decisive battle.

The opposition made new headway in its rapid advance westward through oil towns and along stretches of empty desert highway toward Sirte and beyond to the big prize — the capital, Tripoli.

But the rebels remain woefully outgunned by Gadhafi's forces, who swept the insurgents from positions in eastern Libya until the international intervention forced government troops to withdraw.

Rebels acknowledged they could not have held their ground without international air and cruise missile strikes. Libya state television reported new NATO airstrikes after nightfall, targeting "military and civilian targets" in the cities of Garyan and Mizda about 40 miles and 90 miles respectively from Tripoli.

Speaking in Washington, President Barack Obama declared that the goal of the mission was not to depose Gadhafi, calling that a mistake. He said he would work to replace Gadhafi by "non-military means." He did not elaborate.

Instead, he said, The U.S and its allies moved fast to protect Libya's civilians from imminent massacres by Gadhafi's forces, "and as president, I refused to wait for the images of slaughter and mass graves before taking action."

NATO insisted that it was seeking only to protect civilians and not to give air cover to an opposition march. But that line looked set to become even more blurred. The airstrikes now are clearly enabling rebels bent on overthrowing Gadhafi to push toward the final line of defense on the road to the capital.

Vice Adm. William Gortney, staff director for the Joint Chiefs of Staff, said the U.S. was hitting Libyan targets with Thunderbolts and AC-130 gunships, aircraft that can fly low enough to support ground operations.

There was growing criticism from Russia and other countries that the international air campaign is overstepping the bounds of the U.N. resolution that authorized it. The complaints came at a critical transition in the campaign from a U.S. to a NATO command. That threatens to hamper the operation, as some of the 28 NATO member nations plan to limit their participation to air patrols, rather than attacks on ground targets.

On Monday, rebel fighters moved about 70 miles (110 kilometers) west Monday from the coastal oil terminal and town of Ras Lanouf to just beyond the small town of Bin Jawwad, where their push was halted by government fire along the exposed desert highway and the heavily mined entrance to Sirte.

The rebels are currently just 60 miles (100 kilometers) from Sirte, the bastion of Gadhafi's power in the center of the country.

Take control of that, and there's only the largely rebel-held city of Misrata — and then empty desert — in the way of the capital. Sirte could therefore see some of the fiercest fighting of the rebellion, which began on Feb. 15.

"Gadhafi is not going to give up Sirte easily because straightaway after Sirte is Misrata, and after that it's straight to Gadhafi's house," said Gamal Mughrabi, a 46-year-old rebel fighter. "So Sirte is the last line of defense."

He said there are both anti- and pro-Gadhafi forces inside Sirte.

Some residents were fleeing Sirte, as soldiers from a brigade commanded by Gadhafi's son al-Saadi and allied militiamen streamed to positions on the city's outskirts to defend it, witnesses said. Sirte was hit by airstrikes Sunday night and Monday morning, witnesses said, but they did not know what was targeted.

The city is dominated by members of the Libyan leader's Gadhadhfa tribe. But many in another large Sirte tribe — the Firjan — are believed to resent his rule, and rebels are hoping to encourage them and other tribes there to help them.

"There's Gadhafi and then there's circles around him of supporters. Each circle is slowly peeling off and disappearing," said Gen. Hamdi Hassi, a rebel commander speaking at the small town of Bin Jawwad, just 18 miles (30 kilometers) from the front. "If they rise up, it would make our job easier."

Sirte, which houses a significant air and military base, is crucial both for its strategic position and its symbolic value. Over the years, Gadhafi has made it effectively Libya's second capital, building up what had been a quiet agricultural community into a city of 150,000 with lavish conference halls where Arab and African summits were held.

Fighting in such a densely populated area is likely to complicate the rebels' advance and add to the ambiguity of the NATO-led campaign, authorized by a Security Council resolution to take all necessary measures to protect civilians.

In Russia, which abstained from the U.N. vote, Foreign Minister Sergey Lavrov said strikes on Gadhafi's forces would amount to taking sides in what he called Libya's civil war, and thus would breach the mandate that was initially envisaged as establishing a no-fly zone only to protect civilians.

But the inclusion of language allowing "all necessary means" opened the door to airstrikes and ship-fired cruise missile attacks on Gadhafi's forces to stop attacks on cities and cut supply lines.

And Pentagon officials are looking at plans to expand the firepower and airborne surveillance systems, including using the Air Force's AC-130 gunship armed with cannons that shoot from the side doors, as well as helicopters and drones. That weaponry might allow for more precision in urban fighting, while drawing forces closer to the combat.

NATO's commander for the operation, Lt. Gen. Charles Bouchard of Canada, insisted his mission was clear, saying every decision was designed to prevent attacks on civilians. "Our goal is to protect and help the civilians and population centers under the threat of attack," he said.

Britain and France, which has been the most vocal supporter of the rebellion and is the only Western nation to officially recognize its political leaders, added their voices to those appeals.

In a joint statement, British Prime Minister David Cameron and President Nicolas Sarkozy of France said Gadhafi loyalists should abandon the dictator and side with those seeking his ouster.

"We call on all his followers to leave him before it is too late," the two leaders said. "We call on all Libyans who believe that Gadhafi is leading Libya into a disaster to take the initiative now to organize a transition process."

The Gulf nation of Qatar on Monday recognized Libya's rebels as the legitimate representatives of the country — the first Arab state to do so. Qatar is also one of only two Arab states — the other is the United Arab Emirates — that is contributing fighter planes to the air mission.

Gadhafi is not on the defensive everywhere. His forces continued to besiege Misrata, the main rebel holdout in the west and Libya's third-largest city. Residents reported fighting between rebels and loyalists who fired from tanks on residential areas.

Libyan officials took foreign journalists on a tour of the city's outskirts but not into the center, indicating government control did not extend far. Explosions and gunfire echoed through empty streets lined with burned out tanks and bullet-scarred buildings.

In a late development, The official Tunisian news agency said Libyan Foreign Minister Moussa Koussa arrived in Tunis on what it called a "private visit." No further details were available.

Amnesty International charged Monday that dozens of Libyans have disappeared in recent days, and they are probably in custody. The human rights group said Gadhafi's supporters appear to have a systematic policy to detain anyone suspected of opposition to his rule.

Associated Press writers Hadeel al-Shalchi in Misrata, Libya; Christopher Torchia in Istanbul; and Paula Jelinek and Tom Raum in Washington contributed to this report.