Pro-Moammar Gadhafi fighters fired anti-aircraft guns at revolutionary forces holding the northern gate of a loyalist stronghold for a second day Monday, as frustration with weeks of halting advances grows among the former rebel ranks.

Anti-Gadhafi forces have been massed outside Bani Walid since shortly after Libya's new rulers gained control of Tripoli and other parts of the country in August, leaving just a few major holdouts remaining loyal to the fugitive leader.

The official, trained military of the National Transitional Council, Libya's interim government, pulled away from Bani Walid to regroup and reinforce for a new assault after they were heavily beaten in the city Friday. That has left bands of ragtag, undisciplined fighters on the front line. These include fighters as young as 18 who spend hours smoking hash, shooting at plastic bottles, arguing with one another and sometimes just firing wildly into the streets out of apparent boredom.

When they decide to enter the town, they charge in in half a dozen pickup trucks only to retreat a short while later.

On Monday, three of their cars rode right into an ambush by Gadhafi forces on a street none of the outsiders was familiar with. One of their fellow fighters, Wassim Rajab, said he heard from comrades that four of them were killed.

Describing another typical attempt, fighter Lutfi al-Shibly of Libya's western mountains, said, "We entered the city, 600 meters from the city center, but we didn't have enough forces so we lost the position and had to retreat."

The new leadership is facing a tough fight uprooting the remnants of Gadhafi's regime nearly four weeks after the then-rebels rolled into Tripoli on Aug. 21 and ousted the authoritarian leader.

Bani Walid, 90 miles (140 kilometers) southeast of Tripoli, is just one holdout. Fighting is also raging at Sirte, Gadhafi's hometown on the Mediterranean coast. The regime stronghold of Sabha lies hundreds of miles (kilometers) away in the southern deserts, and there are others deep in the central deserts like the cities of Houn and Zallah.

The battle at Sirte, launched Friday, has also been fierce, but there the revolutionaries have been more organized and have made slow progress.

On Monday, the revolutionary fighters shelled the city with Grad rockets and the sound of gunfights echoed. White smoke wafted over the city, and civilians continued to flee. Gadhafi forces answered back with occasional Katyusha rockets.

Most of the fighters besieging Sirte are from Misrata, a city farther northwest along the coast that survived a brutal weeks-long siege by Gadhafi forces during the civil war. That conflict left them battle-hardened and savvy on the tactics of urban fighting. Regular truckloads of fuel and food arrive from Misrata to keep the fighters supplied outside Sirte.

"We deserve our reputation," said Ali el-Hani, a Misrata native leaning back against his pickup truck mounted with an anti-aircraft gun.

The past three days, they have battled block-by-block into the western side of Sirte, along the beach and along a eucalyptus tree-lined main avenue parallel to the coast. Other fighters in the low hills to the south have been drumming Gadhafi strongpoints in the flat plain of the city below with rockets and mortars. At least two dozen fighters were killed Saturday, but commanders say they gain ground each day. Another revolutionary unit from Benghazi _ farther to the east _ claimed to be fighting its way to Sirte's eastern side to open up a second front.

Late Sunday, Gadhafi spokesman Moussa Ibrahim charged that revolutionary fighters have killed "hundreds every day." He told the Syrian Al-Rai TV station, which has become the Gadhafi mouthpiece, "Sirte is the symbol of resistance in Libya." He did not say where he or Gadhafi were.

He also claimed that Gadhafi fighters captured 17 mercenaries from France, Britain and Qatar near Bani Walid. Britain's Foreign Office said it was aware of Ibrahim's claims, but had no evidence that they were correct.

Perched in the mountains, Bani Walid is far tougher to besiege.

A desert valley called Wadi Zeitoun runs through the center of the city, dividing it into north and south. In the southern part, loyalists command the heights of 100-foot-high escarpments overlooking the valley. Revolutionaries moving in through the city's northern half have reached the edge of the valley several times in the past few days, only to be pummeled by gunfire, mortars and rockets from the other side.

Each time, they retreat back to the relative safety of Wadi Dinar, at the city's northern entrance. The loyalists inside are believed to have received reinforcements and weapons through desert valleys that connect to other Gadhafi-controlled areas.

"Most of the guys here are not from here so it is a big challenge for us to fight," said Walid Turkey, a 28-year-old fighter from Tripoli. "We don't know the streets, and we're learning the makeup as we go along. This causes confusion and chaos."

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Associated Press writer Ryan Lucas in Sirte contributed to this report.