NATO allies on Wednesday agreed to extend their air campaign in Libya by another 90 days, diplomats said, as forces loyal to ousted strongman Muammar Gaddafi maintain resistance.

"NATO agreed to extend (the) mandate for (the) Libya operation," the US ambassador to NATO, Ivo Daalder, wrote on Twitter. The mission "will continue for as long as necessary; end as soon as possible."

An alliance diplomat said that NATO ambassadors agreed on a third 90-day mandate, but the operation could be terminated "at any time" if military commanders deem that civilians are finally safe.

The new mandate was accepted "without any disagreements," the diplomat said, adding that NATO military authorities would provide an update on the situation on the ground every month.

The current 90-day mandate was due to expire on 27 September , but Western leaders have made clear their intention to continue flying NATO warplanes as long as Gaddafi forces harm civilians.

"So long as the Libyan people are being threatened, the NATO-led mission to protect them will continue," US President Barack Obama said Tuesday at a United Nations meeting on Libya welcoming the country's new leadership.

"And those still holding out must understand the old regime is over, and it is time to lay down your arms and join the new Libya," Obama said.

Libya's National Transitional Council took control of Tripoli last month, but Gaddafi forces still control some towns, putting up a fierce resistance in the deposed despot's hometown of Sirte after seven months of fighting.

The new rulers declared victory in the battle for the key southern desert city of Sabha on Wednesday.

But anti-Gaddafi forces have taken heavy losses in the battle for Sirte. Medics say at least 45 NTC fighters have been killed and more than 200 wounded since they launched an offensive last week.

A coalition led by the United States, France and Britain launched the first air strikes against Gaddafi forces on March 19, under a UN mandate to protect civilians from attack.

NATO took over the mission on 31 March after allies ironed out internal divisions.

Germany refused to back the UN resolution that authorised the operation, while France initially refused to hand over command of the mission to NATO.

Only seven nations from the 28-nation alliance are taking part in the air strikes -- the United States, France, Britain, Canada, Denmark, Italy and Belgium. Norway's bombers dropped out of the mission in August.

NATO aircraft have conducted 8,751 missions aimed at idenfifying or hitting targets, according to the alliance's latest figures.