Qaddafi is clinging to power despite a near five-month NATO air campaign, tightening economic sanctions, and a lengthening war with opposition fighters seeking to end his 41-year rule.

The fighters have seized large swathes of the North African state, but are deeply divided and have yet to march on Tripoli.

On Thursday, they reached the village of Bir Shuaib, some 25 km from Zawiyah, which has unsuccessfully risen up against Qaddafi twice this year. It lies less than 50 km west of Tripoli, on the main road to Tunisia, which has been a lifeline for Libya but has begun to crack down on rampant smuggling.

“We’ve gone past Nasr village and right now we’re about 25 km from Zawiyah,” said Faris, a fighter. Reporters were prevented from reaching the front by the fighters to see for themselves.

Fighters in the western mountains do not operate as a single force, as each town has its own command. But when they join forces for major operations they can muster a few thousand men.

Their force is poorly trained and short of heavy weaponry — despite a French arms drop earlier this year — and most analysts do not think they are capable of capturing Tripoli.

In an effort to pile economic and military pressure on Qaddafi, more countries are set to announce next week that they will free frozen assets for the fighters, a British official said.

The senior official said steps taken by the international community meant Qaddafi and his supporters were nearing a “tipping point” when they would be forced from power.

“While it’s hard to predict when this will end, it’s easy to see the pressure is building on Qaddafi and it is only a matter of time before he’s forced to leave power,” the official said.

Britain is playing a leading role in NATO airstrikes against Qaddafi’s forces, which have weakened his armory but failed to allow the rebels deliver a knockout blow.

Tightening the economic noose around Qaddafi, Tunisia said on Thursday its troops were patrolling fuel stations to curb the flow of smuggled gasoline into neighboring Libya.

International sanctions and the effects of Libya’s civil war have disrupted normal supplies of fuel to parts of the country under Qaddafi’s control, but huge volumes of gasoline are instead being smuggled across the Libyan-Tunisian border.

“The armed forces are now conducting checks at fuel stations in the south of Tunisia ... so that neither Tunisians nor Libyans can fill up with large quantities,” Tunisian defense ministry official Mokhtar Ben Nasr told a news conference.

“These checks are aimed at preventing the smuggling of diesel and gasoline to Libya.”

The western battle is one of three widely separated opposition fronts against Qaddafi forces. In the east around the ports of Misrata and Brega, fighting has been bogged down in recent weeks while the western rebels have advanced.

Opposition fighters clashed with forces loyal to Qaddafi in a district of major oil terminal and refinery town, Brega, and hoped to secure the area quickly, a rebel spokesman said.

“There’s an engagement in the residential area of Brega right now. We’re clearing it and hopefully we will finish today,” said spokesman Mohammad Zawawi, who has been predicting a breakthrough for several days.

He said two fighters were killed and 30 wounded on Thursday.

The two sides have been battling for months over Brega, 750 km east of Tripoli. The fighters see securing the town as a tipping point in the nearly six-month-old war and hope to resume oil exports from there as quickly as possible.