Libyan rebel forces bore down on leader Muammar al-Qaddafi’s hometown of Sirte on Monday, advancing with a boost from coalition airstrikes, but U.S. military leaders warn that opposition gains so far are tentative at best.

U.S. Army Gen. Carter Ham has called the rebel gains fragile, an assessment seconded Monday by Vice Adm. William Gortney, director of the military's Joint Staff.

“Clearly the opposition is not well organized and it is not a very robust organization,” Gortney told reporters. “So any gain that they make is tenuous based on that.”

The statement comes amid plans for the United States to scale back its resources dedicated to Libya, just one day after NATO agreed to take over airstrikes in the war-torn country. The USS Providence, a submarine responsible for some of the cruise missiles fired on Qaddafi’s military resources, will no longer support the Libya mission.

“U.S. military participation in this operation is, as we said all along, changing to one primarily of support,” Gortney said in Monday's briefing.

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Opposition forces have seized momentum from the international airstrikes and continued a push westward Monday through oil towns toward the capital of Tripoli.

But the rebels remain woefully outgunned by Qaddafi'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.

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 Qaddafi to push toward the final line of defense on the road to the capital.

On Monday, rebel fighters moved about 70 miles west 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 from Sirte, the bastion of Qaddafi'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.

"Qaddafi is not going to give up Sirte easily because straightaway after Sirte is Misrata, and after that it's straight to Qaddafi'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-Qaddafi forces inside Sirte.

Some residents were fleeing Sirte, as soldiers from a brigade commanded by Qaddafi'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 Qaddafi 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 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, Qaddafi 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 Qaddafi'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 Qaddafi's forces to stop attacks on cities and cut supply lines.

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.

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.

Qaddafi 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.

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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.