Deputy National Security Adviser John Brennan faces members of the Senate Intelligence Committee as he testifies before a hearing on his nomination to be the director of the CIA, on Capitol Hill in Washington, in this Feb 7, 2013 file photo. [Photo/Agencies]

WASHINGTON - The US Senate on Thursday confirmed John Brennan as the Obama administration's next Central Intelligence Agency director, overcoming concerns expressed by Republicans and some Democrats about the administration's use of lethal drone strikes.

After the administration clarified its drone policy, Republican Senators allowed the Senate to vote on Brennan's confirmation, which he won by a vote of 63-34.

Republican Senator Rand Paul spoke for nearly 13 hours on the Senate floor on Wednesday in an attempt to get the administration to declare that "targeted killings" of American citizens on US soil were unconstitutional.

Hours before the final vote, Attorney General Eric Holder sent a brief letter to Paul, saying President Barack Obama does not have the authority to order a drone to kill an American on US soil who was "not engaged in combat".  

Paul, who said his delaying tactic was not a protest against Brennan's qualifications to lead the CIA, called it a major victory for American civil liberties. He nonetheless voted against Brennan.

Democratic Senator Ron Wyden and Republican Senator Marco Rubio, who helped Paul draw attention to Obama's drone program during his lengthy speech, voted in favor of confirming Brennan.

Paul's marathon speech was the latest twist in a convoluted process that delayed Brennan's Senate confirmation for weeks.

The nomination became a vehicle for Republicans and some Democratic critics to pressure the White House to disclose sensitive government records.

These included emails and documents related to targeted killings and the administration's response to the attack last year on a US diplomatic compound in Benghazi, Libya.

Holder initially declined to declare the targeted killings would be unconstitutional, saying there could be situations, similar to the Japanese attack on Pearl Harbor in 1941 and the September 11 al-Qaida attacks on New York and Washington in 2001, in which such killings might be appropriate.

Ted Cruz, another Republican senator who tried to slow Brennan's confirmation, said that under questioning, Holder would only declare that such killings were inappropriate, but he eventually acknowledged they would be unconstitutional.

The administration has increasingly used drone strikes to target militants overseas, particularly in Pakistan and Yemen.

The administration is under pressure from some members of Congress to be more transparent about its rules and legal justifications for the use of drones against both American citizens and foreigners, an issue Brennan likely will have to deal with from his new office at CIA headquarters.

Another issue near the top of Brennan's agenda will be the CIA's response to a 6,000-page investigative report by the Senate Intelligence Committee into controversial detention, interrogation and "extraordinary rendition" operations which it conducted during the administration of George W. Bush.

The report is said by sources familiar with the investigation to be highly critical of these agency operations.

But the committee did not interview any witnesses during the course of its investigation and some former officials knowledgeable about the CIA operations argue its conclusions are tendentious.