US President Barack Obama prepared on Monday to give a prime-time address to the nation aimed at winning the support of a war-weary American public for the military intervention in Libya.

After inheriting wars in Afghanistan and Iraq, the Nobel Peace Prize-winning President finds himself embroiled in yet another conflict, this time one of his own choosing and one that many at home are not convinced by.

In his address at 7.30 pm (2320 GMT) from the National Defense University in Washington, Mr Obama must win over a largely skeptical public preoccupied by domestic economic concerns and unclear what the Libya endgame is.

Mr Obama, who has been criticized since military action began nine days ago for not getting his message across, will also give interviews on Tuesday with the anchors of three US television networks, the White House said.

Lawmakers, including many from Mr Obama's own Democratic Party, are angry that Congress was not consulted before troops were deployed and have raised concerns that the Libya mission is ill-defined and the exit strategy unclear.

On the eve of one of the president's most important foreign policy speeches to date, Defense secretary Robert Gates and secretary of state Hillary Clinton took to the Sunday news talkshows to answer his critics.

They argued that the United States had to intervene quickly in Libya for humanitarian reasons and also made a broader case that inaction would have had disastrous knock-on effects for the region.

"It was not a vital national interest to the United States, but it was an interest," Gates told ABC's "This Week" program, pointing to the possibility of a mass exodus of refugees that could have overwhelmed Tunisia and Egypt.

"So you had a potentially significantly destabilizing event taking place in Libya that put at risk potentially the revolutions in both Tunisia and Egypt.

"Egypt is central to the future of the West Asia," Gates added.

Speaking alongside him, Clinton urged critics to ask themselves where things would be now if the United States had not intervened.

"Imagine we were sitting here and Benghazi had been overrun, a city of 700,000 people, and tens of thousands of people had been slaughtered, hundreds of thousands had fled. Either with nowhere to go or overwhelming Egypt while it's in its own difficult transition," she said.

"The cries would be, 'why did the United States not do anything? How could you stand by when, you know, France, and the United Kingdom, and other Europeans, and the Arab League, and your Arab partners were saying, 'You've got to do something?'"

The pair also sought to allay fears that the United States would get bogged down in a protracted conflict in Libya by highlighting the limited scope of the mission and, in particular, of the US role in that mission.

Mr Obama's position was made easier Sunday when Nato ambassadors agreed in Brussels to take command of all Libyan operations, but the United States was still clearly bearing the brunt of the military burden.

On the ground in Libya rebel forces were gaining momentum, as, aided by coalition air strikes, they wrested back control of key towns from Gaddafi's retreating ground forces and pressed on westwards towards Tripoli.

The United States and its allies are hopeful that Gaddafi's regime will eventually crack and dread a long drawn-out conflict that could test the political will of their patchwork coalition.

"Don't underestimate the potential for elements of the regime themselves to crack," Gates told CBS's "face the nation" program.

Clinton said the allies would be encouraging defections through a UN special envoy, former Jordanian foreign minister Abdul Ilah Khatib, who would visit Tripoli and ask Gaddafi: "'Do you really want to be a pariah? Do you really want to end up in the international criminal court?"

Clinton joins foreign ministers from 35 other countries Tuesday in London to discuss the Libya conflict before she and Gates can expect tough questioning behind closed-doors from congressional committees on Wednesday.

US public support for the conflict is lukewarm: a March 22 Gallup poll showed that 47 percent of Americans surveyed approve of action against Libya, less than for most US military campaigns over the past four decades.