Rebels waging a drawn-out war to oust Libyan leader Muammar Gaddafi have run out of money and a chief rebel has accused the West of not meeting promises to deliver urgent financial aid.

His appeal came as cracks were appearing in the NATO alliance over its three-month bombing campaign against Gaddafi, with some showing mission fatigue and the US accusing some European allies of failing to pull their weight.

The rebels have made important gains on various fronts in the past few weeks, but remain far from seizing Gaddafi's powerbase of Tripoli and its hinterland.

At least eight rebels were killed in fighting near the northwestern town of Nalut as insurgents seek to press an advance into Gaddafi's heartland that has proven slow despite weeks of NATO air strikes on their behalf.

The gun battles in the village of Takut, just outside Nalut, today followed exchanges of heavy artillery fire near the city of Zlitan, on the other side of Tripoli, as the insurgents tried to take government-held territory to the east of the city.

The remarks by rebel oil chief Ali Tarhouni highlighted the insurgents' struggle to make ends meet with war damage to energy infrastructure in their eastern territory having knocked out oil production there.

Western powers are assisting the rebels through daily air strikes on forces loyal to Gaddafi and have pledged to expand aid by tapping into Libyan assets frozen abroad.

But Tarhouni, also the insurgents' finance minister, said there had been no follow-through on such promises.

The economy in eastern Libya, where much of the oil that once made Libya a major OPEC exporter came from, is in shambles.

Rebel leaders are struggling to find cash to pay for military operations and salaries in a society where, thanks to the legacy of Gaddafi's centralised rule, most people rely on state wages.

The European Union has pledged financial infusions and the United States, which took a leading role in securing a UN backed no-fly zone over Libya, has promised more aid and offers of loans to keep the rebels afloat.

The rebels are trying to seal off coastal Tripoli from the east, west and south but their advances have been halting and weeks of NATO strikes pounding Gaddafi's compound and other targets have failed to bring down his 41-year-old rule.