By Andrew Quinn and Steve Holland
WASHINGTON (Reuters) - The Obama administration strongly backed a U.N. vote on Thursday for a no-fly zone and other help for Libyan rebels, hoping to reassert its faltering leadership on the crisis despite misgivings among U.S. military planners and key lawmakers.
The Security Council vote could thrust the U.S. military into risky new overseas action and follows a dramatic pivot by Washington, which had been accused of moving too slowly to support Libyan rebels as Muammar Gaddafi's forces looked poised to snuff out their uprising.
The United States joined Britain, France and Lebanon in sponsoring the resolution, which authorized "all necessary measures" on Libya and passed by a vote of 10-0 with five abstentions on the 15-member council.
U.S. officials said the Arab League's call for U.N. authorization of a no-fly zone was key, and Gaddafi's strengthening position on the ground helped build support within the administration for the aggressive new steps.
But the Pentagon -- already stretched by operations in Iraq and Afghanistan -- has doubts and lawmakers worry the United States may be saddled with a new military mission that has no clear endpoint short of Gaddafi's ouster.
"There was a genuine policy debate within the administration over whether to do this or not. Whether it opens up more permanent rifts it's hard to say," said Mark Quartermain, a scholar at the Center for Strategic and International Studies.
French diplomatic sources said military action could come within hours of the U.N. vote, and might include France, Britain and possibly the United States and one or more Arab states. But a U.S. military official said no immediate U.S. action was expected following the decision.
While other nations or NATO may play roles in any eventual military action against Libya, U.S. officials expect the United States with its extensive air and sea assets would do the heavy lifting in a campaign that could include air strikes against Libyan tanks and artillery.
JUST IN TIME OR TOO LATE
The Libyan opposition has appealed for immediate help to prevent its stronghold of Benghazi from falling to forces loyal to Gaddafi. The question now facing President Barack Obama and other world leaders is whether the action they are planning is too little, too late.
Obama and Secretary of State Hillary Clinton have both said the U.S. goal is to see Gaddafi removed -- but it is unclear if the military options now on the table can achieve that, and if so, at what cost.
The Pentagon voiced concerns on Thursday about U.S. military engagement in Libya, echoing recent comments from Defense Secretary Robert Gates, and U.S. Air Force General Norton Schwartz who said it was " "overly optimistic" to say a no-fly zone could be set up in a few days.
Nevertheless, the Obama administration decided to act after concluding the Libyan opposition was a positive force and that Gaddafi's rising stream of threats indicated a real threat to the United States beyond the unfolding humanitarian crisis inflicted on his people.
"If Gaddafi is successful, you also face a number of other considerable risks," Undersecretary of State William Burns told a Senate panel on Thursday, saying those included "the danger of him returning to terrorism and violent extremism," and fueling turmoil across the Middle East.
Burns said the United States could support alternatives "short of boots on the ground" to achieve its aims in Libya.
Michele Dunne, a Middle East scholar at the Carnegie Endowment for International Peace, said the balance of factors weighing in favor of intervention simply tipped.
"This was not an easy or simple mission of any kind. It is simply I think that the administration finally decided that the alternative looked much worse," Dunne said.
Republican Senator Richard Lugar, one of the party's top foreign policy experts, expressed concern on Thursday over the costs of military action, the risk of escalation, the uncertain reception by Arabs, and the strains on the U.S. military - reflecting worries among both Republicans and Democrats.
Middle East expert Brian Katulis at the Center for American Progress said coming days would show whether the administration had "defined down" its goal of removing Gaddafi, but that the fact Washington decided to intervene was key.
"Even if it saves one life, I think it's really important," Katulis said. "What's been going on this week has been not only ensuring the solidity of the support, but then also trying to talk about what sorts of contributions various countries might make toward achieving some sort of cessation of hostilities in Libya."
(Additional reporting by Susan Cornwell, Caren Bohan and Missy Ryan; Editing by Peter Cooney)