Republican Rand Paul stiff-armed President Barack Obama's CIA nomination Wednesday, drawing support from fellow senators concerned about the administration's refusal to rule out drone strikes on US soil.

With the blocking tactic stretching into it's sixth hour, Senate Majority Leader Harry Reid tried to bring Paul's filibuster to a close, but the freshman from Kentucky refused, continuing his hold on any Senate action as he railed against US policy on targeted killings.

Paul said he would be happy to yield "if the president or the attorney general will clarify that they are not going to kill non-combatants in America."

The drone issue gained fresh currency on Capitol Hill with senators from both parties pressing US Attorney General Eric Holder on whether the administration believes such drone attacks could be justified.

Paul began by saying he will "speak until I can no longer speak," and demanding answers from President Barack Obama on the secret unmanned aerial drone program that has emerged as the most contentious element of John Brennan's nomination to head the spy agency.

"I will speak as long as it takes until the alarm is sounded from coast to coast that our Constitution is important, that your rights to trial by jury are precious, that no American should be killed by a drone, on American soil, without first being charged with a crime, without first being found guilty by a court," Paul said.

Brennan's nomination easily cleared the Senate Intelligence Committee Tuesday despite Republican concerns about lack of White House transparency on last year's attack in Benghazi, Libya that killed four Americans.

The Senate Democratic leadership wanted to move this week on a confirmation vote for Brennan.

But Paul was making the Senate leadership sweat a little. By the filibuster's third hour, he enlisted Republicans including Mike Lee, Ted Cruz and Saxby Chambliss. By hour four Marco Rubio joined in, as did Democrat Ron Wyden, who has long questioned White House power on national security issues.

"I think you are raising some of the most important questions... we could be asking," Wyden said, without causing Paul to yield control of the floor.

"This is just the beginning of this debate."

Paul had threatened to filibuster Brennan's nomination as early February 13, when he demanded answers from the administration about the president's power to authorize lethal force.

Holder responded Tuesday, stressing that while Obama had "no intention" of ordering drone strikes on US soil, the scenario could be possible if there was an "extraordinary circumstance" such as an attack similar to 9/11.

Paul reacted with revulsion.

"When I asked the president, can you kill an American on American soil, it should have been an easy answer.... It should have been a resounding and unequivocal 'no,'" Paul said Wednesday.

"The president's response? He hasn't killed anyone yet."

"Is that enough? Are we satisfied by that?" Paul asked.

And he insisted that his critique was not partisan. "Were this a Republican president, I'd be here saying exactly the same thing."

Paul acknowledged that US drone strikes have proved effective in places like Pakistan and Yemen, including a strike on US-born radical preacher Anwar al-Awlaki, whom Paul branded a traitor.

But "if you're going to kill non-combatants, people eating dinner, in America, there have to be some rules."

Paul began his filibuster at 11:47 am (1637 GMT), and he was going strong more than six hours later, recalling the classic "talking filibusters" of old.

But he acknowledged Brennan was a virtual lock for the CIA, calling the filibuster "a blip" in the process.

The ultimate goal, he said, was to get Obama to "say explicitly that non-combatants in America would not be killed by drones."

The issue became the focal point at a Wednesday hearing where Republican Cruz confronted Holder about drones.

Cruz asked Holder whether the Constitution would allow a drone strike on someone sitting at a cafe "if that individual is not posing an imminent and immediate threat of death or bodily harm."

Holder said "I would not think that that would be an appropriate use of any kind of lethal force."

That prompted a fierce response from Cruz, who said it was "remarkable" that the administration could not state unequivocally that such a strike would not be legally allowed.