Nominated to head the CIA, John Brennan told a protest-disrupted Senate confirmation hearing Thursday the United States employs drone strikes only as a deterrent against imminent terrorist threats, not as punishment for previous actions, firmly defending the controversial attacks that have killed three Americans and an unknown number of foreigners.

"Nothing could be further from the truth," he said of the idea that the U.S. uses the strikes by unmanned aircraft as retaliation in the broad fight against terrorism.

On another thorny topic, under sometimes-combative questioning from senators, he conceded that after years of intelligence work he is uncertain whether the use of waterboarding in interrogations has yielded valuable information.

He declined several times to say whether waterboarding is torture, but he did say it is "something that is reprehensible and should never be done again."

In hours of questioning from the Senate Intelligence Committee, Brennan made repeated general pledges to increase the flow of information to members of the panel, but he was less specific when it came to individual cases. Asked at one point whether he would provide a list of countries where the CIA has used lethal authority, he replied, "It would be my intention to do everything possible" to comply.

At another point, he said he had no second thoughts about having opposed a planned strike against Osama bin Laden in 1998, a few months before the bombings of two U.S. embassies. The plan was not "well-grounded," he said, adding that other intelligence officials also recommended against proceeding. Brennan was at the CIA at the time.

Underwear bomb

Brennan was questioned extensively about leaks to the media about an al-Qaeda plot to detonate a new type of underwear bomb on a Western airline. He acknowledged trying to limit the damage to national security from the disclosures.

On May 7 of last year, The Associated Press reported that the CIA thwarted an ambitious plot by al-Qaeda's affiliate in Yemen to destroy a U.S.-bound airliner, using a bomb with a sophisticated new design around the one-year anniversary of the killing of Osama bin Laden. The next day, the Los Angeles Times reported that the would-be bomber was cooperating with U.S. authorities.

During Thursday's hearing Republican Senators James Risch of Idaho and Dan Coats of Indiana were among those who contended Brennan had inadvertently revealed that the U.S. had a spy inside Yemen's al-Qaeda branch when, hours after the first AP report appeared, he told a group of media consultants that "there was no active threat during the bin Laden anniversary because ... we had inside control of the plot."

Brennan acknowledged the comments about "inside control" but denied that they revealed any secret elements associated with the U.S. operation.

"I think I have the leak right here," Risch said.

Bristling, Brennan shot back, "I disagree with that vehemently."

Waterboarding and 'enhanced interrogation'

Brennan is a veteran of more than three decades in intelligence work, and is currently serving as Obama's top counter-terrorism adviser in the White House. Any thought he had of becoming CIA director four years ago vanished amid questions about the role he played at the CIA when the Bush administration approved waterboarding and other forms of "enhanced interrogation" of suspected terrorists.

In a statement at the beginning of Thursday's session, Brennan said the United States remains at war with al-Qaeda and other terrorists and is under "daily cyberattack" by foreign countries and others.

He said historic transformations continue sweeping through the Middle East and North Africa, with "major implications for our interests, Israel's security, our Arab partners and the prospects for peace and stability throughout the region." Additionally, he said that Iran and North Korea "remain bent on pursuing nuclear weapons and intercontinental ballistic missile delivery systems."

The hearing was interrupted repeatedly — once before it began and then several times before Brennan had completed his preliminary remarks. At one point, Senator Dianne Feinstein, a Democrat from California, and the panel's chairman, briefly ordered the proceedings halted and the room cleared so those re-entering could be screened to block obvious protesters.

The shouted protests centered on CIA drone strikes that have killed three American citizens and an unknown number of foreigners overseas.

It was a topic very much on the mind of the committee members who eventually will vote on Brennan's confirmation.

Obama releases classified report

In the hours before the hearing began, U.S. President Barack Obama ordered that a classified paper outlining the legal rationale for striking at U.S. citizens abroad be made available for members of the House and Senate intelligence panels to read.

It was an attempt to clear the way for Brennan's approval, given hints from some lawmakers that they might hold up confirmation unless they had access to the material.

White House spokesman Jay Carney said Wednesday the White House is making "extraordinary accommodation" in allowing lawmakers to view classified Justice Department legal advice on drone strikes against Americans. Carney said the White House does not plan to send the Justice memos to legislators beyond those on the House and Senate intelligence committees.

Responding to the assurances from the administration, Senator Ron Wyden, a Democrat from Oregon, said he was encouraged when Obama called him on the telephone to inform him of his decision.

However, Wyden said that after he went to read the material he became concerned the Department of Justice "is not following through" on the commitment. He asked Brennan to look into the matter, and the CIA nominee said he would.

Drone strikes

Despite the sometimes-combative questioning, Brennan's confirmation seemed a foregone conclusion as he appeared before the committee. "I look forward to working with you," said Senator Mark Udall, a Democrat from Colorado.

Wyden made the drone strikes the main focus of his time to question Brennan, asking at one point what could be done "so that the American people are brought into this debate and have a full understanding of what rules" are for their use.

An undated handout photo provided by the U.S. Air Force shows a MQ-9 Reaper. The Reaper is one of the primary drones used by the CIA to strike al-Qaeda and Taliban targets. (Lt. Col. Leslie Pratt, US Air Force/Associated Press)

Brennan said the day's hearings were part of that effort, and said he backs speeches by officials as a way to explain counter-terrorism programs. He said there is a "misimpression by the American people' who believe drone strikes are aimed at suspects in past attacks. Instead, he said, "we only take such actions as a last resort to save lives" when there is no other alternative in what officials believe is an imminent threat.

Aides have portrayed Brennan as cautious in the use of drones, restraining others at the CIA or military who seek to use them more often. At the same time, as the White House's counterterror adviser, he has presided over an explosion of drone strikes in Pakistan, Yemen and Somalia.

Fewer than 50 strikes took place during the Bush administration, while more than 360 strikes have been launched under Obama, according to the website The Long War Journal, which tracks the operations.

Daily attacks on U.S. computer systems

Administration officials say Brennan would further limit the use of drones by the CIA and leave the majority of strikes to the military. Brennan signaled in his written answers that he would not seek to expand the CIA's paramilitary operations. In written responses to questions from the committee in advance of the hearing, Brennan wrote, "While the CIA needs to maintain a paramilitary capability ... the CIA should not be used, in my view, to carry out traditional military activities."

The CIA's drone strikes primarily focus on al-Qaeda and Taliban targets in the tribal regions of Pakistan. The agency also carries out strikes in Yemen, where three American citizens with al-Qaeda connections have been killed: Anwar al-Awlaki, his 16-year-old-son and Samir Khan.

On the question of waterboarding, Brennan said that while serving as a deputy manager at the CIA during the Bush administration, he was told such interrogation methods produced "valuable information." Now, after reading a 300-page summary of a 6,000-page report on CIA interrogation and detention policies, he said he does "not know what the truth is."

In his opening statement, he said U.S. computer systems are under daily attack by "nation states, international criminal organizations, subnational groups and individual hackers."