Obama also asserted that those who perpetrate violence against civilians would be held accountable  [GALLO/GETTY]

US President Barack Obama has insisted that Libyan leader Muammar Gaddafi "step down from power and leave," his most explicit statement of support for rebels challenging Gaddafi's four-decade rule in a region convulsed by uprisings against authoritarian regimes.

"We will continue to send a clear message: The violence must stop. Muammar Gaddafi has lost the legitimacy to lead and he must leave," Obama said at a White House news conference with Mexican President Felipe Calderon on Thursday.

Obama also appeared to suggest that Gaddafi loyalists switch sides in support of the revolutionaries.

"Those around him have to understand that violence that they perpetrate against innocent civilians will be monitored and they will be held accountable for it," Obama said. "And so to the extent that they are making calculations in their own minds about which way history is moving, they should know history is moving against Colonel Gaddafi."

The US administration has been tempering tough talk on Libya with a dose of reality, explaining that even a no-fly zone to control the skies over the country would require a military attack. Two leading senators on defence matters responded on Thursday by urging a strong US stance aiding Gaddafi's opposition.

No-fly zone

The Pentagon is making it clear it does not want war, even as senators passed a resolution urging the UN to act on setting up a no-fly zone.

Read more of our Libya coverage

Robert Gates, the US defence secretary, illustrated the administration's effort to rein in "loose talk" about military options to force Gaddafi from power in a statement on Wednesday.

It was an acknowledgement that, short of an unlikely military offensive by a US-led coalition, the options for international action to stem the violence are highly limited.

"Let's just call a spade a spade: A no-fly zone begins with an attack on Libya to destroy the air defences," Gates told a congressional panel.

The Pentagon could get the job done if ordered by the president, he said, but he noted that an attack would require more air power than a single US aircraft carrier, which typically carries about 75 planes. "It is a big operation in a big country," Gates said.

Senator John McCain, a leading Republican, took issue with Gates' comment about "loose talk," saying on Thursday he believed it was well within the US military's capabilities to shoot down Libyan aircraft.

"May I just say, personally I don't think it's 'loose talk' on the part of the people on the ground in Libya or the Arab League or others, including the prime minister of England, that this option should be given the strongest consideration," McCain said.

Endorsing McCain's remarks, Senator Joe Lieberman, an independent, said another option was to provide US air defence weapons to the Libyan rebels and to train them in their use.

Source: Agencies