By Andrew Quinn and Mark Hosenball
WASHINGTON (Reuters) - President Barack Obama on Friday set the terms of Washington's limited involvement in Libya's crisis to protecting civilians but stopping short of ousting embattled leader Muammar Gaddafi.
Shortly after Obama spoke, U.S. officials said Gaddafi's forces continued to advance toward the eastern rebel stronghold of Benghazi, violating the U.N. demand for a ceasefire.
Obama, delivering an ultimatum to Gaddafi, said the United States would work with its partners to enforce U.N. demands for a ceasefire but promised no U.S. ground troops would be deployed in the oil-producing North African country.
Although Obama has called on Gaddafi to leave, he stressed the United States would not use its power beyond a well-defined goal: "specifically the protection of civilians in Libya."
Obama's decision put the United States on track for a new and uncertain conflict in the Middle East just as many in Congress and the U.S. public fret over stretched U.S. financial and military resources.
Obama demanded that Gaddafi withdraw his forces in the eastern part of the country, where they threaten to overwhelm opposition strongholds such as Benghazi.
"All attacks against all civilians must stop," Obama said, vowing to enforce the U.N. Security Council resolution approved on Thursday that authorizes a "no-fly" zone and other steps to help rebels under attack.
"These terms are not subject to negotiation," Obama said, adding Gaddafi must also reconnect gas, water and power supplies to rebel-held towns.
In an interview on CNN television, U.N. Ambassador Susan Rice said that Gaddafi, whose forces have been advancing on the rebel-held eastern city of Benghazi, was in violation of a U.N. Security Council resolution adopted Thursday, which called for an immediate ceasefire and banned all flights over Libya.
Facing misgivings among military planners over any large U.S. engagement, Obama said he was driven by concern Gaddafi could commit atrocities if allowed to quash the rebellion, which could further destabilize the entire Middle East.
Obama underscored that the United States was working with key European allies as well as Arab countries, hoping to dispel fears that the United States was embarking on a risky adventure without adequate backing.
Secretary of State Hillary Clinton will travel to Paris on Saturday to take part in an international meeting on the next steps on Libya.
Recent U.S. polls have shown 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.
Clinton said earlier the immediate goal was stopping violence against civilians but the long-term objective was to see Gaddafi depart, although Obama himself did not mention this in his remarks.
"The first and overwhelmingly urgent action is to end the violence," she said, saying "a final result of any negotiations would have to be the decision by Colonel Gaddafi to leave."
Obama's comments came after a meeting with key lawmakers, some of whom have voiced concern the United States was lurching toward another open-ended conflict in a Muslim country during period of unprecedented turmoil across the Middle East.
While many voiced to support for the move, some even in Obama's Democratic party said the United States should let others lead the way.
"I firmly believe that our European allies and the members of the Arab League must take the leading role," said Steny Hoyer, the Democratic whip in the House of Representatives.
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.
A European national security official told Reuters that a White House meeting on Monday gave impetus to the harder line with U.S. officials proposing a "no-drive" ban requiring Gaddafi to stop ground movements of his forces.
Pentagon officials said they were ready to act on Libya but declined to discuss possible operations. U.S. officials announced they would deploy additional amphibious ships to the Mediterranean as part of plans for responding to situation in Libya.
Former officials and analysts said the Pentagon may be hoping that limited air strikes may induce Gaddafi to rein in his forces without the need for greater U.S. involvement, which U.S. military planners do not want.
"The Pentagon does not want to get involved in this," said one U.S. national security official familiar with recent discussions about a possible Libya operation.
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.
(Additional reporting by Steve Holland, Caren Bohan, Patricia Zengerle and Missy Ryan; Editing by Paul Simao and Eric Walsh)