Top Pentagon officials strongly suggested Tuesday that American military intervention in Libya is unlikely, even as Secretary of State Hillary Rodham Clinton told a congressional panel that the Obama administration was looking at "every single lever it can use" to push Moammar Gadhafi out of power.

At a Pentagon news conference, Defense Secretary Robert Gates said he had ordered two Navy amphibious warships into the Mediterranean Sea, along with an extra 400 Marines, in case they are needed to evacuate civilians or provide humanitarian relief. And while he did not rule out other options such as providing air cover for Libyan rebels, he made clear that he has little enthusiasm for direct military intervention.

Gates noted that the U.N. Security Council sanctions resolution passed last week provided no authorization to use armed force in Libya, nor is there agreement among NATO allies on taking military action.

Asked specifically about establishing a "no-fly" zone over Libya, Gates said this and other frequently suggested military actions "have their own consequences" for U.S. interests — not just in Libya but throughout the greater Middle East.

"And we also have to think about, frankly, the use of the U.S. military in another country in the Middle East," Gates added, referring to the long war in Iraq and its backlash in the Arab world. "So I think we're sensitive about all of these things, but we will provide the president with a full range of options."

The Senate weighed in on the issue later Tuesday, unanimously passing a non-binding resolution calling on the U.N. Security Council to impose a no-fly zone over Libya. The resolution condemned the "gross and systematic violations of human rights in Libya" and demanded that Gadhafi leave office.

Clinton spoke in more dramatic terms about prospects for U.S. and international intervention to protect Libyans from their own government. She warned that Libya is at risk of collapsing into a "protracted civil war" amid increasingly violent clashes with anti-government rebels.

"The stakes are high," she told the House Foreign Affairs Committee.

The U.S. must lead an international response to the crisis, including expanding already tough financial and travel sanctions against Gadhafi, his family and confidants and possibly imposing a no-fly zone over Libya, she said.

"The United States continues to look at every single lever it can use against the Gadhafi regime," she said.

On a cautionary note, Clinton said outside military intervention also might compromise Gadhafi's opponents who do not want to be seen as serving foreign troops.

"We are also very conscious of the desire by the Libyan opposition forces that they be seen as doing this by themselves on behalf of the Libyan people, that there not be outside intervention by any external force, because they want this to have been their accomplishment," Clinton told the committee. "We respect that."

As she spoke, Gadhafi and forces still loyal to him sought to protect their remaining strongholds in and around the capital of Tripoli and take back rebel-held areas in the east.

Clinton said U.S. officials were aware that defecting military officers were attempting to organize fighters to defend areas they hold and "even try to take Tripoli away from Col. Gadhafi." Gates, however, said the U.S. has an incomplete picture of the rebels' potential for prevailing on their own. Nor was it clear how many civilians have died, he added.

"The honest answer is that we don't know in that respect, in terms of the number of casualties, in terms of the potential capabilities of the opposition," he said. "We're in the same realm of speculation pretty much as everybody else."

Unlike in Egypt, where senior U.S. officials had regular contact with Egyptian military leaders during mass demonstrations that led to the ouster of President Hosni Mubarak's government last month, the U.S. and Libya have no substantial military-to-military relationship.

More broadly, Gates said he is optimistic that the convulsions rocking authoritarian regimes throughout the greater Middle East will prove beneficial.

"I think, first of all, these revolutions in Tunisia and in Egypt and the protests elsewhere that are leading to reforms in a number of governments, I think, are an extraordinary setback for al-Qaida," he said of the terrorist organization. "It basically gives the lie to al-Qaida's claim that the only way to get rid of authoritarian governments is through extremist violence. And the peoples of several countries in the region are proving this not to be the case."

In her testimony, Clinton said that protective military air cover in Libya is a possibility, although she and the top U.S. military commander in the Middle East said it would have drawbacks.

"There are arguments that would favor it, questions that would be raised about it, but it is under active consideration," Clinton said.

Testifying before a separate panel, Marine Gen. James Mattis, head of U.S. Central Command, said the military would have to take down Libyan air defenses in order to establish a no-fly zone there.

Mattis told the Senate Armed Services Committee that in his view, setting up a no-fly zone would be "challenging."

"You would have to remove the (Libyan) air defense capability in order to establish the no-fly zone. So, no illusions here, it would be a military operation. It wouldn't simply be telling people not to fly airplanes."

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, made a similar point.

"It's an extraordinarily complex operation to set up," Mullen said while appearing with Gates at the Pentagon. He and Gates stressed that no decisions have been made about U.S. military action, and that their aim is to preserve as many options as possible for President Barack Obama as the crisis evolves.

Among other potential risks in Libya is the use of suspected stockpiles of mustard gas against the rebels.

"Our information is that security around those things has been increased," Gates told reporters. "And I think I'd just leave it as we're keeping an eye on it, and I think it is not an immediate concern for us."


Associated Press writers Lolita C. Baldor and Donna Cassata contributed to this report.