NEWS FLASH

WASHINGTON (Reuters) – Top U.S. officials on Sunday stressed the limits of American military involvement in Libya, despite a heavy assault on Muammar Gaddafi's forces, and said Libyans would decide their country's fate themselves.

White House National Security Adviser Tom Donilon said Libyan leader Gaddafi had "lost legitimacy" and was isolated but the focus of military action was on protecting Libyan civilians, not ousting the veteran ruler from power.

Defense Secretary Robert Gates, speaking as he flew to Russia, said the U.S. will not have a "preeminent role" in the coalition that will maintain a no-fly zone over Libya, and expected to turn over "primary responsibility" for the mission to others within days.

Britain or France could take charge of the air operation, or NATO could lead, if sensitivities among the Arab League over working under the Western alliance leadership were assuaged.

Gates spoke amid growing concern among U.S. politicians over the scope and nature of the Libya mission and after an acknowledgement from the top U.S. military officer that the assault on Gaddafi's forces could lead to an impasse.

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But Donilon, who spoke to reporters while traveling with U.S. President Barack Obama in Brazil, said Washington and its allies had "a very good first day" in the Libya mission.

The United States says the U.N.-endorsed intervention is aimed at forcing Gaddafi's troops into a ceasefire and ending attacks on civilians who launched an uprising last month.

Obama has called in recent weeks for Gaddafi to step down but U.S. officials have emphasized that is not the goal of the United Nations authorized attacks on Libya. The United States is eager to avoid similarities to the 2003 invasion of Iraq and ouster of President Saddam Hussein.

Forces loyal to Gaddafi, who has ruled Libya for 41 years, had been taking back large swathes of territory from rebels until the air attacks, which may give the rebels the chance to regroup. Military analysts say it is unclear if they can.

"I think this is basically going to have to be resolved by the Libyans themselves," said Gates. "Whether or not there is additional outside help for the rebels remains to be seen."

Admiral Mike Mullen, chairman of the U.S. military's Joint Chiefs of Staff, told the CBS program "Face the Nation" the air mission in the North African country has a clear, limited scope.

STALEMATE POSSIBLE

But Mullen said the outcome of military action in Libya was "very uncertain." Asked if it could end in a stalemate with Gaddafi, Mullen replied: "I don't think that's for me to answer. Certainly, I recognize that's a possibility."

Gates said the United States wanted Libya to remain a unified country, saying partition into a rebel-held east and Gaddafi-controlled west "would be a real formula for enduring instability."

The aerial assault by U.S., French and British planes has halted an advance by Gaddafi's armored units on the rebel-held city of Benghazi and attacks on air defenses and radar sites have allowed the ad-hoc Western coalition to establish "a consistent and persistent" air presence over Libya, enforcing a no-fly zone, a U.S. official said.

Vice Admiral Bill Gortney, director of the U.S. military's Joint Staff, told reporters there had been no new Libyan air activity or radar emissions and had been a significant decrease in Libyan air surveillance since strikes began Saturday.

Benghazi was not yet free from threat, said Gortney, but Gaddafi's forces in the area were in distress and "suffering from isolation and confusion" after the air assaults.

A Libyan army spokesman declared a fresh ceasefire earlier on Sunday but the United States and Britain said they did not believe Gaddafi was honoring it and said they would continue to enforce the no-fly zone.

PRESSING FOR CLARITY

Senior Republicans pressed President Barack Obama to give a clear rationale for the Libya mission, reflecting concern that U.S. forces could get bogged down in a long-running, costly operation that lacks defined goals.

"The administration has a responsibility to define for the American people, the Congress and our troops, what the mission in Libya is," House of Representatives Speaker John Boehner, the top Republican in Congress, said in a statement.

"I am concerned that the use of military force in the absence of clear political objectives for our country risks entrenching the United States in a humanitarian mission whose scope and duration are not known," added Republican Representative Howard McKeon, chairman of the House Armed Services Committee.

The United States is now fighting in three conflicts -- Iraq, Afghanistan and Libya -- while struggling under a huge budget deficit and national debt. The Pentagon also has plans to cut $78 billion in defense spending over five years.

(Additional reporting by Phil Stewart, Alister Bull, Mark Hosenball, Missy Ryan and Vicki Allen; Writing by Alistair Bell and Sean Maguire; editing by Todd Eastham)