The administration of President Barack Obama has come under mounting pressure to arm rebels facing an emboldened and regrouping military, amid charges Washington missed recent chances to oust Libya's strongman.

Obama has insisted that all options, including military action, remain on the table with respect to Libya, where Muammar Gaddafi's forces have unleashed deadly airstrikes on rebels and civilians in efforts to crush an uprising in which thousands are feared dead.

But with the administration cautioning that a decision on a no-fly zone was still far off, US lawmakers and former officials appearing coalesced around the likelihood that supplying weapons to the outgunned rebels was a way forward.

"I assume that a lot of weapons are going to find their way there (to rebels in Libya) from one means or another over the course of the next weeks," Senate foreign relations committee chairman John Kerry, from Obama's Democratic Party, told CBS's 'Face the Nation' on Sunday.

Ex-governor of New Mexico Bill Richardson, a former US ambassador to the United Nations, also said it was time to 'covertly arm the rebels' and enforce a no-fly zone over Libya.

And Stephen Hadley, national security adviser to Obama's predecessor George W. Bush, said Washington should look at the potential for funneling arms to Gaddafi's opponents.

"Obviously, if there is a way to get weapons into the hands of the rebels, if we can get anti-aircraft systems so that they can enforce a no-fly zone over their own territory, that would be helpful," Hadley said.

Pentagon spokesman Colonel David Lapan declined to confirm any potential plans to send weapons to opposition forces, simply telling that 'all options are being considered'.

Kerry said a no-fly zone should be set up in conjunction with allies, but warned that direct military action would be 'trickier'.

"The last thing we want to think about is any kind of military intervention, and I don't consider the fly zone stepping over that line," Kerry said.

"We don't want troops on the ground. They don't want troops on the ground."

Defense Secretary Robert Gates has warned that imposing a no-fly zone begins with direct military action, as it would require bombing raids to eradicate Libya's air defenses, thus potentially dragging the United States into a third major war front after Iraq and Afghanistan.

But both Kerry and Republican Senator John McCain - two of the most renowned combat veterans in the US Senate - downplayed the risk and complexity of such a move.

"That's actually not the only option for what one could do," said Kerry. "One could crater the airports and the runways and leave them incapable of using them for a period of time."

There were other ways of displaying US might to Tripoli, including the use of military transport planes to fly Egyptian refugees out of Tunisia, and the recent arrival in the Mediterranean of two US warships with marines on board.

"We have made the presence of American military felt for that purpose," Kerry said.

A former Tripoli regime member complained that Washington has missed a key opportunity to end Gaddafi's four-decade grip on power.

"We asked for help when he was on the ropes," said Libya's ex-minister of immigration Ali Errishi, who resigned shortly after the uprising began nearly three weeks ago, along with several key Gaddafi loyalists and military figures.

"They were dragging their feet, I don't know why."

He also stressed he had 'no doubt' Gaddafi would refuse to negotiate terms for his own departure.

"This is a man who has shown that there's only one choice for Libyan people: either I rule you or I kill you," Errishi added.

Rebels have taken control of much of Libya's eastern half, but Gaddafi's well-armed forces have gone on the counter-attack against rag-tag groups of rebels who are often armed only with AK-47 assault rifles.

McCain, Obama's 2008 rival for the presidency, said a no-fly zone would 'send a signal to Gaddafi' that Obama was serious in his call for the Libyan leader to step down.

"It would be encouraging to the resistance, who are certainly outgunned from the air," he told ABC's 'This Week'.

"We can't risk allowing Gaddafi to massacre people from the air."

He backed off from direct military engagement but noted Washington could also provide technical assistance and intelligence capabilities.