Image caption Libyan rebels have seized control of several towns from forces loyal to Col Muammar Gaddafi

Britain and France have urged supporters of Libya's Muammar Gaddafi to "drop him before it is too late".

The countries' leaders issued a statement hours before the international community is to meet in London to discuss Libya's future.

The statement said the Gaddafi regime had completely lost its legitimacy and the leader must "go immediately".

The coalition enforcing a UN-backed no-fly zone over Libya has denied it is to provide cover for a rebel advance.

In Libya, the rebel army has been moving rapidly westwards, but came under heavy attack on Monday on the approach to Col Gaddafi's birthplace of Sirte.

Despite earlier claims the rebels had captured Sirte, they ran into ambushes after taking Bin Jawad, about 100km (62 miles) to the east. The rebels said they had pulled back after coming under heavy machine-gun fire.

Tuesday's conference will bring together about 40 delegations, with all members of the coalition in the military operation, as well as the UN, Nato, the African Union and Arab League.

In a joint statement on Monday, UK Prime Minister David Cameron and French President Nicolas Sarkozy said the conference would "bring the international community together to support Libya's transition from violent dictatorship and to help create the conditions where the people of Libya can choose their own future".

Speaking directly to the people of Libya, the statement said the Gaddafi regime had completely lost its legitimacy and the leader must "go immediately".

"We call on all his followers to leave him before it is too late. We call on all Libyans who believe that Gaddafi is leading Libya into a disaster to take the initiative now to organise a transition process," it said.

Meanwhile, Mr Sarkozy and Mr Cameron held a conference call with US President Barack Obama and German Chancellor Angela Merkel to discuss a Franco-British proposal to help pave the way for a political transition, the French presidency said.

'Beneficial effect'

Coalition strikes continued on Monday. The Ministry of Defence (MoD) said RAF Tornados had destroyed two tanks and two armoured vehicles with Brimstone missiles.

Earlier, British jets had launched missiles against ammunition bunkers in the Sabha area of southern Libya.

We have stopped the assault on Benghazi and helped to create conditions in which a number of towns have been liberated from Gaddafi's onslaught David Cameron, UK Prime Minister

The MoD said initial reports suggested the bunkers, being used by Col Gaddafi's government to attack the opposition-held city of Misrata, had been destroyed.

Speaking in the House of Commons before Monday's strikes, Mr Cameron praised the "courageous" work of British pilots who destroyed 22 tanks and armoured vehicles at the weekend.

He said RAF pilots had flown more than 120 sorties and completed over 250 hours of flights as part of the international action in Libya.

The allies intervened after the UN Security Council passed a resolution urging the protection of Libyan civilians, amid the uprising against Col Gaddafi's rule.

Mr Cameron said the allied operations had had a "significant and beneficial effect" over the past 10 days.

"We have stopped the assault on Benghazi and helped to create conditions in which a number of towns have been liberated from Gaddafi's onslaught," he said.

"The no-fly zone is now fully operational and effective. When it has been challenged, Gaddafi's planes have been shot down. He can no longer terrorise the Libyan people from the air."

Later, Foreign Secretary William Hague insisted the UK and its allies would stick to the UN resolution that ruled out any occupation.

"There will be no occupation force, there will be no invasion force from the UK or any of our coalition partners," he told BBC News.

"We think Gaddafi should go, but then almost the whole world thinks that he should go."