By David Morgan

WASHINGTON (Reuters) - The top U.S. military officer acknowledged on Sunday that a no-fly zone over Libya could create a stalemate with Muammar Gaddafi's forces even as Western warplanes halted the Libyan leader's anti-rebel offensive at Benghazi.

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

But Mullen said the end-game of military action in Libya is "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."

"It's very uncertain on how this ends," he added.

The aerial assault by U.S., French and British planes stopped an advance by Gaddafi's troops on the rebel-held city of Benghazi and killed at least 14 government soldiers.

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

Libya's armed forces have issued a command to all units to observe an immediate ceasefire, a Libyan army spokesman said.

Forces loyal to Gaddafi, who has ruled Libya for 41 years, have taken back large swathes of territory from rebels but the air attacks may help the rebels regroup.

Senior Republicans in Congress pressed U.S. President Barack Obama to explain the U.S. mission in Libya.

"The president is the commander-in-chief. But 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 at this point and cannot be controlled by us," added Republican Representative Howard McKeon, chairman of the House Armed Services Committee.

Obama has called in recent weeks for Gaddafi to step down but the United States is eager to avoid the impression it is trying to overthrow him by force, like the 2003 invasion of Iraq and ouster of President Saddam Hussein.

"The opposition needs breathing room to solidify. This operation buys time. ... This isn't going to be pretty," a U.S. national security official told Reuters.

BENGHAZI STRIKES

Mullen told NBC's "Meet the Press" program that the U.S.-led air strikes "took out" Gaddafi's air defenses, struck air fields and attacked Libyan ground forces near the eastern rebel stronghold of Benghazi.

"We have halted him in the vicinity of Benghazi, which is where he was most recently on the march," he said, adding that Western forces had established combat air patrols over the city that would be extended westward toward Tripoli over time.

Mullen said Western military operations are narrowly focused on protecting civilians and aiding humanitarian efforts.

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.

Senator Joe Lieberman, an independent and a leading U.S. proponent of the no-fly zone, called for Gaddafi to be removed from office.

"Once the president of the United States says, as President (Barack) Obama did, that Gaddafi must go, if we don't work with our allies to make sure Gaddafi does go, America's credibility and prestige suffers all over the world," Lieberman told CNN.

Senator Lindsey Graham, a Republican, told the "Fox News Sunday" program the mission should be to get rid of Gaddafi, saying it should "isolate, strangle and replace this man."

France sent an aircraft carrier toward Libya and its planes were over the country again on Sunday, defense officials said. Britain said its planes had targeted Libya's air defenses mainly around the capital Tripoli.

Italian aircraft are ready to join operations against Libya starting Sunday, Defense Minister Ignazio La Russa said.

In the next few days, Mullen said, the United States expects to relinquish its leadership of the operation, dubbed "Odyssey Dawn." But he did not say who would assume the lead.

The U.S. role would then shift to support operations including intelligence, signal jamming, aerial refueling and humanitarian efforts.

U.S. Navy Growlers provided electronic support while AV-8B Harriers from the 26th Marine Expeditionary Unit conducted air strikes against Gaddafi's ground forces and air defenses.

Missile strikes launched by the United States and Britain as part of a bid to cripple Libyan air defenses hit 20 of 22 targets, the U.S. military said. The military also said three U.S. stealth bombers took part in airstrikes early on Sunday.

(Additional reporting by Mark Hosenball, Missy Ryan, Vicki Allen and Caren Bohan; Writing by Alistair Bell; Editing by Will Dunham)