Opposition forces have launched a two-pronged offensive in Western Libya, increasing pressure to isolate Libyan leader Muammar Gaddafi's stronghold of Tripoli.

Opposition fighters advanced toward the towns of Gharyan and Az-Zawiyah on Saturday, attempting to cut off the southern coastal route from Tunisia that Gaddafi uses for supplies.

Early in the day, rebel fighters claimed victory in Gharyan after Gaddafi's soldiers withdrew. However, regime forces returned several hours later and clashes continued.

Al Jazeera's Zeina Khodr, who is in the south of Az-Zawiyah, said opposition fighters claim "they have managed to take 70 per cent of the town, despite the threat of snipers still in the area."

The gains are possible "because the Gaddafi forces' defences were weak and that fighters received help from inside the city. As they expected, residents took up arms and fought alongside them when they arrived.

"The town had previously risen up against Gaddafi, but government forces quelled that uprising.

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Libyan government spokesman Moussa Ibrahim rejected the claims: "Az-Zawiyah is completely under our control. A very small group of rebels tried to enter from the south of Az-Zawiyah but they were stopped easily by our armed forces."

Rebel forces launched ground attacks after NATO planes hit targets in these areas.

Rebels also said they gained ground on Saturday in the government-held oil town of Brega.

"We have taken control of residential area number 2, after number 3 yesterday," out of four zones, a military spokesman, Mohammed Zawiwa, told AFP. "The pipe factory has fallen into our hands today."

The rebels' claims of taking over Brega were also denied by government officials.

Opposition forces hope that by taking complete control of the city, its oil terminal and sea port will allow them to resume oil exports.

Capture of Tawurgha

On the western front, opposition commanders said they had control of the town of Tawurgha as they pushed to cut supply routes to forces loyal to Gaddafi.

In a symbolic show of victory, fighters tore down green flags that had been hoisted atop buildings by Gaddafi supporters who had occupied the area.

"Gaddafi is finished!" shouted a jubilant 31-year-old fighter named Mohammed. "There is no way back. We have taken Tawurgha!"

The rebels encountered heavy fighting and sizable pockets of resistance among a maze of buildings and date palms.

Al Jazeera's Andrew Simmons, reporting from Tawurgha, said it was a heavily co-ordinated operation with NATO, with six tanks involved.

"Fighting is going on in the old quarter of the town where Gaddafi forces are still putting up some resistance," he said.

"Opposition fighters have been searching houses one after the other with green flags.

"Many, many Gaddafi forces have been arrested in areas surrounding the town to try to secure the area to stop Grad missiles from being fired on Misrata from here.

"Their other objective is to try to cut the supply line to Gaddafi stronghold of Sirte."

The citizens of Misrata have blamed forces in Tawurgha for many of the attacks on their hometown.

The embattled Libyan leader has clung to power despite five months of NATO air strikes, suffocating economic sanctions and an expanding war with opposition forces.