FF building up their forces east of Sirte, in preparation for a major assault. They continue to face unexpectedly stiff resistance.

@rafasemueveRafael E. Cartagena

#NATO preparing bomb attack on Gadafi´s birthtown #Sirte to protect civilians... but, what if civilians support Gadafi there? #Libya

British Tornado jets fired precision-guided missiles at a large bunker in Libyan leader Col Muammar Gaddafi's hometown of Sirte, officials have said.

The aircraft took off from RAF Marham in Norfolk on a long-range strike mission on Thursday night.

The Libyan rebels are building up their forces east of Sirte, in preparation for a major assault. They continue to face unexpectedly stiff resistance.

Rebel leaders are meanwhile calling on other states to unfreeze Libyan assets.

The UN has already agreed to release $1.5bn (£1bn) in funds, which had been frozen under Security Council sanctions at the beginning of the uprising in February, to help with immediate humanitarian needs.

But South African President Jacob Zuma said the African Union (AU) would not yet recognise the rebel National Transitional Council (NTC) as the sole representative of the Libyan people until hostilities ended.

"If there is fighting, there is fighting. So we can't stand here and say this is the legitimate [government] now. The process is fluid. That's part of what we inform countries - whether there is an authority to recognise," he said.

Closing in

In a statement issued on Friday, the UK Ministry of Defence said "a formation of Tornado GR4s... fired a salvo of Storm Shadow precision-guided missiles against a large headquarters bunker" in Sirte.

The bunker housed a command and control centre. There is no indication that Col Gaddafi was in Sirte, which is 250 miles (400km) east of Tripoli, or in the bunker itself at the time of the attack.

"It's not a question of finding Gaddafi, it's ensuring the regime does not have the capability to continue waging war against its own people," UK Defence Secretary Liam Fox told the BBC.

"The attack that we launched on the bunker in Sirte last night was to make sure that there was no alternative command and control should the regime try to leave Tripoli."

Nato warplanes also targeted 29 vehicles mounted with weapons near Sirte and bombed surface-to-air missile facilities near Tripoli, the alliance said at a daily briefing in Brussels.

Meanwhile, the rebels are building up their forces around the oil port town of Ras Lanuf, preparing for an assault on Sirte, about 250km (155 miles) along the coast to the west.