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US President Barack Obama has defended the first military intervention of his presidency, insisting US involvement in Libya will be limited.

He said US participation in the coalition had saved "countless lives", but that overthrowing Muammar Gaddafi by force would be a mistake.

Delegates from dozens of countries are gathering in London for a conference on the future of Libya.

The rebel advance there has been halted near Col Gaddafi's birthplace, Sirte.

Anti-Gaddafi forces had made rapid progress westwards from their stronghold in Benghazi in recent days, greatly aided by international air strikes.

But rebel fighters said pro-Gaddafi forces had used heavy weaponry to check their advance some 50km (30 miles) east of Sirte, says the ³ÉÈË¿ìÊÖ's Ben Brown in Ajdabiya.

In eastern Libya, rebel radio has been urging more people in the west of the country to join the anti-Gaddafi uprising.

While Nato insists it is impartial in the conflict, Russia has renewed its expressions of concern, saying intervention in an internal civil war is not sanctioned by UN Security Council Resolution 1973.

Ahead of Tuesday's conference, British Foreign Secretary William Hague said he wanted Col Gaddafi to leave power and face trial at the International Criminal Court.

Some 40 delegations - from the coalition, the UN, Nato, the African Union and Arab League, but not the Libyan government - will be represented in London. Rebel officials have been invited for talks on the meeting's sidelines, although not to the conference itself.

In a letter to those attending the conference, Col Gaddafi called for an end to the "barbaric offensive" on his country.

In his first televised address on the Libyan intervention, Mr Obama said that having led the initial campaign, the US would hand over to Nato allies on Wednesday.

"We have stopped Gaddafi's deadly advance," he said

But the lead in enforcing the no-fly zone and protecting civilians on the ground would now move to the Americans' allies, he added.

"Because of this transition to a broader, Nato-based coalition, the risk and cost of this operation - to our military, and to American taxpayers - will be reduced significantly," Mr Obama said.

"We must always measure our interests against the need for action," the president continued. "But that cannot be an argument for never acting on behalf of what's right."

Earlier on Monday, in a video conference with French President Nicolas Sarkozy, German Chancellor Angela Merkel and UK Prime Minister David Cameron, Mr Obama had agreed that Col Gaddafi "had lost any legitimacy to rule and should leave power, and that the Libyan people should have the political space to determine their own future", the White House said.

An Italian proposal to end the crisis includes offering Col Gaddafi an escape route from Libya, ensuring a quick ceasefire and facilitating dialogue between rebels and tribal leaders.

Foreign Minister Franco Frattini said he had discussed the proposals with Germany and France.

Libyan Deputy Foreign Minister Khaled Kaim, meanwhile, has called on those attending Tuesday's conference in London to act as "peacemakers, not warmongers".

In recent days, anti-Gaddafi forces have seized a number of coastal communities and important oil installations, including Ras Lanuf, Brega, Uqayla and Bin Jawad.

However, repeated attacks by government troops have prevented them reaching the symbolic target of Sirte.

A Pentagon spokesman in Washington, Vice Admiral Bill Gortney, said that because the Libyan rebels were not well organised, any military gains they made would be tenuous.

He said the rebels were clearly benefiting from actions of the US, which has started using heavily-armed low-flying aircraft against government forces.

Nato has denied that its air strikes are meant to provide cover for a rebel advance.