Libyan rebels came under heavy fire as they pushed toward Moammar Gaddafi’s home town on the Mediterranean coast Monday, setting up a potentially crucial battle in the six-week-old uprising.

The rebels said they had captured the towns of Nawfaliyah and Harawah in their advance west, but Gaddafi’s birthplace of Sirte remained in government hands. Fighting continued throughout the day in Wadi al-Ahmar, a valley east of Sirte with terrain that may pose more challenges than the flat desert roads the rebels have seized so far.

Along the coastal road, the oil terminals at Brega and Ras Lanuf were operating, after rebel forces retook them over the weekend. Gaddafi forces had wrested the two towns from rebel control two and a half weeks ago.

Rebels sped up the road bringing supplies to the front line or waited in long lines at gas stations. They were jubilant about their latest victories, which have come amid coalition airstrikes against Gaddafi targets.

“We’re going to Sirte. Anyone who doesn’t love Gaddafi, come along!” yelled a young man standing on the back of a pickup truck as it sped west.

“Yeah, yeah, yeah,” cried another one, flashing a victory sign. “Amrika, Amrika, Amrika!”

A senior U.S. military official told reporters that the coalition’s sustained airstrikes had forced Libya’s ground troops to retreat and that opposition forces had moved to within 80 miles of Sirte. “We believe the regime is preparing to dig in at Sirte, setting up a number of checkpoints and placing tanks throughout the city,” said Vice Adm. William Gortney, director of the Pentagon’s Joint Staff.

There was also heavy fighting in the strategic city of Misurata, particularly near the center of the city.

Even as the rebels made gains, senior U.S. military officials cautioned that the opposition lacked the firepower of the Libyan government forces and could be quickly overrun if the airstrikes ceased. “The opposition is not well organized, and it is not a very robust organization,” Gortney said. “That’s obvious. So any gain that they make is tenuous based on that. Clearly, they’re achieving benefit from the actions that we’re taking.”

The U.S. and allied forces flew 107 strike sorties over the past 24 hours, an increase of about 20 over the previous day and a sign that the air campaign aimed at Gaddafi’s ground forces continues to build.

Families that had fled Ras Lanuf in early March, when Gaddafi’s troops entered the city, were trickling back Monday. Some had spent the past weeks in tents in the desert.

“We were afraid of looting, so we came back,” said Abdurabu el-Maghrabi, 32, standing in front of his home. Looking down his mostly empty street, he said he did not know what had happened to most of his neighbors. “It’s really hard to know any news right now because everyone has left,” he said.

A rebel soldier lying on a gurney with a bandaged leg said he had seen about 10 pickup trucks full of government troops and a large number of rebel fighters in Wadi al-Ahmar on Monday. The two sides were evenly matched, he said, and were using machine guns, rocket-propelled grenades and antiaircraft missiles to battle for the valley.

The soldier, Abdulatif Sagluf, a 29-year-old health and safety engineer from the rebel capital of Benghazi, said he had been accidentally bayoneted that morning by a fellow rebel. He said the rebel forces were “getting more organized day by day.”

International response

In Paris, French President Nicolas Sarkozy and British Prime Minister David Cameron said in a joint statement Monday that “Gaddafi must go immediately,” and they urged his supporters to “drop him before it is too late.”

But Russian Foreign Minister Sergei Lavrov accused Western powers of exceeding the bounds of a March 17 U.N. Security Council resolution that authorized a no-fly zone over Libya and the protection of civilians.

“We consider that intervention by the coalition in what is essentially an internal civil war is not sanctioned by the U.N. Security Council resolution,” Lavrov told reporters. He cited reports of “coalition strikes on columns of Gaddafi’s forces, reports about support for actions by the armed insurgents.”

U.N. Secretary-General Ban Ki-Moon and Secretary of State Hillary Rodham Clinton were meeting in London Tuesday with foreign ministers from the Arab League and dozens of other countries to plot out an endgame for Gaddafi’s tottering regime and to strike agreement on plans for Libya’s future.

The Persian Gulf emirate of Qatar, meanwhile, became the first Arab country to formally recognize the rebels’ Transitional National Council as Libya’s legitimate government. The move came a day after the council, based in Benghazi, announced a contract with Qatar Petroleum to market crude oil produced from eastern Libyan oil fields no longer under Gaddafi’s control.

Libyan state television condemned Qatar’s recognition, calling it a “blatant interference” in Libya’s affairs.

Jaffe reported from Washington.