By Andrew Quinn and Caren Bohan
WASHINGTON (Reuters) - The United States moved toward potentially deep and risky engagement in Libya's bloody political crisis on Friday, vowing to remove defiant leader Muammar Gaddafi and halt violence against embattled rebels.
"We don't know what the final outcome will be. The first and overwhelmingly urgent action is to end the violence," Secretary of State Hillary Clinton said as Washington and its allies prepared to act on a U.N. Security Council vote authorizing "all available means" to stop Gaddafi.
President Barack Obama met key lawmakers at the White House to discuss Libya and was due to make a public statement at 2 p.m. EST which may set out the rationale for yet another U.S. military mission involving the Muslim world amid a period of unprecedented turmoil across the Middle East.
The shift toward a tougher U.S. stance in favor of military action followed an extended internal debate within the Obama administration over how to stop Gaddafi from routing rebels fighting to end his four-decade rule.
Clinton said the international community would move carefully on Libya, working first to stop Gaddafi's loyalists from attacking opposition forces in the east of the country but ultimately ensuring that the longtime Libyan strongman is removed from power.
"It is important that we take this one step at a time," Clinton told reporters, saying the Security Council vote on Thursday underscored the two basic goals promoted by the United States.
"Number One: stop the violence, and Number Two: we do believe that a final result of any negotiations would have to be the decision by Colonel Gaddafi to leave," she said.
With Gaddafi's forces pressing their advantage against the beleaguered opposition, the United States on Wednesday abandoned early caution over calls to impose a "no-fly zone" over Libya and pushed hard at the U.N. Security Council for an even broader mandate that officials said could include air strikes on Libyan tanks and artillery.
Pentagon officials said they were ready to act on Libya orders but declined to discuss possible operations.
France and Britain, which joined the United States in pushing for the Security Council vote, said they were ready to act, perhaps backed by one or two Arab states. Most analysts expect the United States, with its superior air and sea assets, to eventually become involved.
The Obama administration faces serious misgivings, both in Congress and among military planners, over the wisdom of involvement in Libya with U.S. forces already fighting a war in Afghanistan and engaged in Iraq.
A U.S. national security official familiar with current military planning and operations said there was enormous resistance among military commanders to any U.S. involvement in any kind of operation against Gaddafi's forces in Libya.
"The Pentagon does not want to get involved in this," said the official, who is familiar with recent discussions about a possible Libya operation which have been taking place in both Washington and among commanders in the region.
The official said that commanders were asking the Obama administration, "What do you want to get out of this ?" but had not gotten a clear answer.
The U.S. public, too, appears lukewarm. Polls taken last week showed more than half of respondents opposed U.S. action on Libya, with much smaller numbers supporting it.
"The administration hasn't mobilized the American public nor the Congress to support U.S. military intervention," said Peter Feaver, a Duke University professor and former national security advisor to President George W. Bush.
He said the administration's mixed messages on Libya had left it's position unclear.
BROAD INTERNATIONAL SUPPORT
The new move on Libya comes as Washington struggles to assess the wave of political changes across the Middle East that have toppled U.S.-allied leaders in Egypt and Tunisia and rocked other U.S. allies in Yemen and Bahrain -- both key security partners for the United States.
U.S. officials stress that they worked to build broad international support for their approach to Libya, pointing to an Arab League decision to endorse a no-fly zone over Libya as a crucial turning point.
But any expanded U.S. military action on Libya could have unexpected consequences. The U.S. intelligence chief last week suggested Libya could fracture into several de facto states, while Clinton herself suggested it was at risk of becoming "a giant Somalia" that could give new haven to al Qaeda and other extremists.
Political analysts said that delays in mustering international backing for the Libya action may have made the job more difficult -- which could force the United States into more extensive engagement than it would like.
"This is obviously more difficult now than it would have been a few weeks ago given the setbacks that Gaddafi has delivered to the rebels," said Robert Danin, a Middle East expert at the Council on Foreign Relations.
"The U.S. has now crossed a very significant and potentially very dangerous threshold. We are now getting involved in Libya in a serious way."
(Additional reporting by Mark Hosenball and Tabassum Zakaria, editing by Anthony Boadle)