The US Senate confirmed John Brennan as CIA director Thursday after the White House doused a fierce debate over the potential use of domestic drones, which could have delayed filling the crucial post.

Brennan, a counter-terrorism expert and 25-year veteran of the CIA who was the architect of the controversial secret program that uses armed drones in "targeted killings" overseas, was given the green light after a two-month confirmation process that heated up in its final week.

The Senate voted 63-34 with several Republicans approving Brennan following a dramatic 13-hour stall tactic, called a filibuster, the night before by Senator Rand Paul, who demanded clarification from the White House on the limits of the government's legal power to kill its own citizens on US soil.

Obama welcomed the vote, praising Brennan's "determination to keep America safe, his commitment to working with Congress, his ability to build relationships with foreign partners and his fidelity to the values that define us as a nation."

The Brennan confirmation fills a crucial position on Obama's national security team, after senators gave their nod to John Kerry to lead the State Department and to Chuck Hagel as Pentagon chief.

Despite his reservations, Paul was among several Republicans who allowed the Brennan vote to go ahead, although he ultimately voted against Brennan's confirmation.

The senator from Kentucky had delayed the nomination, seeking clarification from the White House over whether it was US policy to allow the killing by a drone strike of a "non-combatant American citizen on US soil."

He got his answer Thursday, when Attorney General Eric Holder wrote to tell Paul that this was not US policy.

Paul, who emerged from his filibuster as somewhat of a political celebrity with backing from some fellow Republicans, said that receiving Holder's letter proved that "the entire battle was worthwhile."

The unmanned aerial drone program had emerged as the most contentious element of Brennan's nomination to head the CIA.

Earlier this week Holder had said 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 acknowledged that US drone strikes have proven effective in places like Pakistan and Yemen, including a deadly strike on US-born radical preacher Anwar al-Awlaki, whom Paul branded a traitor.

But he added during his filibuster: "If you're going to kill non-combatants, people eating dinner, in America, there have to be some rules."

The New America Foundation estimates that US drones have conducted about 350 attacks since 2004, most of them under Obama, who ramped up the program after taking over the White House from his predecessor George W. Bush.

Between 1,963 and 3,293 people were killed by the strikes, including 261 to 305 civilians, according to the think-tank.

Obama stressed that "timely, accurate intelligence is absolutely critical to disrupting terrorist attacks, dismantling Al-Qaeda and its affiliates, and meeting the broad array of security challenges that we face as a nation."

Brennan replaces David Petraeus, who resigned suddenly from the top CIA job in November after it was revealed he had had an extramarital affair with his biographer.

Among the Republicans voting for Brennan was Senator John McCain, who took issue with Paul's grandstanding on drones the night before.

McCain took to the Senate floor Thursday to rebuke his colleague, saying Paul should know "that the US government can not randomly target American citizens on US soil or anywhere else."

"If Mr. Paul wants to be taken seriously, he needs to do more than pull political stunts.... He needs to know what he's talking about."

McCain also blasted as "ridiculous" Paul's comments on how the US government might have even wanted to target actress and activist Jane Fonda for having spoken out against US policy during a wartime visit to Hanoi, Vietnam.

"That's one thing if you want to try her for treason," Paul said during his filibuster, "but are you going to just drop a drone, a Hellfire missile on Jane Fonda?"