Charles Levinson reports from Zawiya:

Fighting raged for a fourth straight day in this strategic coastal city 30 miles west of Tripoli on Wednesday, as rebel fighters battled to mop up pockets of loyalist soldiers and laid siege to the regime’s last working oil refinery.

The roads leading to Zawiya were clogged with rebel fighters pouring toward town and refugees fleeing in the opposite direction. The refugee flow from Zawiya, a city of 200,000 people, and from Tripoli hinted at the possible flood that could pour into rebel-held havens and neighboring Tunisia as fighting moves closer to the capital.

Rebel fighters reported a series of other gains in the west as well. They said they had eliminated government forces in the town of Tiji and had secured the coastal town of Sabratha, west of Zawiya, bolstering their control of the vital coast road that is a key lifeline to the capital, Tripoli, which is still controlled by Col. Moammar Gadhafi.

The regime wasn’t available for comment. Officials in Tripoli have been quiet in recent days, not holding the regular news conferences and journalist trips they had sponsored earlier in the conflict.

While the momentum has swung toward the rebels in recent weeks, they remain heavily dependant upon air power from the North Atlantic Treaty Organization, whose 90-day mandate to operate in Libya could need to be extended a second time in late September. And the rebels’ potentially toughest battle—to capture Tripoli—may still lie ahead.

Zawiya’s fleeing residents and rebel fighters said most of the city was now under rebel control. On Wednesday, fighting was fiercest around the oil refinery on the western edge of the city, where a group of pro-Gadhafi fighters were holed up and surrounded by rebel fighters, said rebel commanders and fighters.

The commanders said they had already shut down the fuel lines leading to and from the refinery, knocking Mr. Gadhafi’s last refinery offline. The refinery provides only a fraction of the regime’s fuel oil and doesn’t produce gasoline. The government has been smuggling in most of its fuel needs from Algeria and Tunisia.

Still, it further pressures Col. Gadhafi and deals him a symbolic blow by depriving the once oil-rich strongman of the ability to produce even a drop of his own fuel needs.

Control of the coast road between Tunisia and Zawiya, and another road from Algeria in the south are far more crucial fuel and supply conduits for the regime, but are also collapsing beneath the rebel offensive.