Libyan rebels said they were less than 20 miles (30 kilometers) from Moammar Gadhafi's main stronghold of Tripoli on Sunday, a day after opposition fighters launched their first attack on the capital itself.

Fighters said a 600-strong rebel force that set out from Zawiya has reached the outskirts of the village of Jedaim and was coming under heavy fire from regime forces on the eastern side of the town.

Murad Dabdoub, a fighter who returned to Zawiya from the front, told The Associated Press that Gadhafi's forces were pounding rebel positions with rockets, mortars and anti-aircraft fire.

The rebels' arrival at Jedaim was also confirmed by Abdul-Bari Gilan, a doctor in Zawiya. He told the AP that he had treated a rebel who was wounded in the fighting at Jedaim.

NATO said the shifting battle lines and concentration of fighting in towns and villages are making it more difficult to identify and engage targets for airstrikes.

"It's much tougher to do in an urban area," NATO spokesman Col. Roland Lavoie said. "This requires very precise and deep intelligence to achieve without endangering the civilian population."

An AP reporter in Tripoli, meanwhile, said the city was largely quiet on Sunday after a night of gunfire and explosions.

Libyan rebels said Saturday that they had launched their first attack on Tripoli in coordination with NATO and gunbattles and mortar rounds rocked the city. NATO aircraft also made heavy bombing runs after nightfall, with loud explosions booming across the city.