WASHINGTON: A Republican senator seeking to block President Barack Obama's pick to lead the CIA over concerns about drone strikes on US soil completed a nearly 13-hour speaking filibuster early Thursday.

In a remarkable display highlighting the partisan rift in Washington, Rand Paul led the filibuster of John Brennan's appointment after the White House refused to unequivocally rule out drone strikes on US soil.

Senate Majority Leader Harry Reid, a Democrat, tried to bring the blocking tactic to a close, but Paul, a favorite of the Republican Party's conservative Tea Party faction, refused to stop.

His oratory held up any other Senate action as he railed against US policy on targeted killings from just before noon (1700 GMT) Wednesday until nearly 1:00 am (0600 GMT) Thursday.

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

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

Paul demanded answers from President Barack Obama on the secret unmanned aerial drone program that has emerged as the most contentious element of Brennan's nomination to head the Central Intelligence Agency.

"I rise today to begin to filibuster John Brennan's nomination for the CIA," he said at 11:47 am (1647 GMT).

"I will speak until I can no longer speak. 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."

He finally yielded the floor at 12:38 am (0558 GMT), to a round of applause.

"I've discovered that there are some limits to filibustering and I'm going to have to take care of one of those in a few minutes here," he joked.

Brennan's nomination easily cleared the Senate Intelligence Committee Tuesday, despite fury from leading Republican lawmakers at what they said was a lack of disclosure over last year's attack on the US mission in Benghazi, Libya, which killed four Americans, including US ambassador Chris Stevens.

The Senate Democratic leadership wanted to move this week on a confirmation vote for Brennan, but Paul was making Senate leaders sweat a little.

At close to the 12-hour mark, Senate Minority Leader Mitch McConnell joined the filibuster, saying: "I intend to oppose the nomination and congratulate my colleague from Kentucky for this extraordinary effort."

Paul had earlier enlisted Republicans Mike Lee, Ted Cruz and Saxby Chambliss. Rising Republican star Marco Rubio also joined, as did Democrat Ron Wyden, who has long questioned White House power on national security issues.

"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."

Tea Party-backed Cruz, noting that Paul might be unaware of the goings on in the outside world, told Paul that Twitter was "blowing up" over his filibuster and proceeded to read out more than a dozen tweets in support of his colleague.

Congressman Louie Gohmert walked over from the House of Representatives to give Paul some encouragement and hand him some candy while Cruz was speaking.

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 were an "extraordinary circumstance" such as a 9/11-style attack.

Paul reacted with revulsion, saying he had expected an "unequivocal 'no.'"

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

Paul acknowledged that US drone strikes have proven 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," he added.

Paul's effort recalled the classic "talking filibusters" of old, famously portrayed in the fictional 1939 classic, "Mr. Smith Goes to Washington," starring Jimmy Stewart.

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