FILE - In this Oct. 29, 2010 file photo, Deputy National Security Adviser for Homeland Security and Counterterrorism John Brennan briefs reporters at the White House in Washington. Brennan, now President Barack Obama's nominee to be CIA director, withdrew from consideration for the job in 2008 amid criticism over the agency's use of harsh interrogation techniques, like waterboarding, against terrorist suspects. This time, in 2013, he's making it clear he strongly opposes such practices. Former and current U.S. intelligence officials say Brennan wasn't so vocal a decade ago. (AP Photo/Charles Dharapak, File)

Washington - President Barack Obama's top counterterrorism adviser, John Brennan, is expected to face tough questioning about the administration's policy of using drone attacks to kill U.S. terrorism suspects during his Senate confirmation hearing on Thursday, the first step toward his confirmation as CIA director.

On Wednesday, Obama directed the Justice Department to provide congressional intelligence committees access to a classified opinion laying out the legal basis for armed drone strikes on U.S. citizens overseas alleged to be involved in terrorist plots, an administration official said.

Brennan, 57, who was a top CIA official under former President George W. Bush, has helped oversee the policy.

Although there has been no groundswell of opposition to Brennan's confirmation, he is expected to be examined closely about U.S. spy activities from waterboarding to the use of drones at the Senate Intelligence Committee hearing, scheduled for 2:30 p.m. EST (1930 GMT).

Some of the most intense questioning will likely be from liberal Democrats, not the conservative Republicans who have raised the strongest objections to some of Obama's other nominees, including Chuck Hagel, his choice to lead the Pentagon.

The White House said the release of the documents reflected Obama's understanding of the importance of questions about the drone program.

“The president takes these issues very seriously and he believes that the conversation about this is valued and that the questions about it are legitimate,” White House spokesman Jay Carney told reporters.

Civil liberties groups have criticized the program as effectively a green light to assassinate Americans without due process in the courts under the U.S. Constitution.

Brennan is expected to win confirmation from both the panel and later the full U.S. Senate.

Senator Ron Wyden, a member of the intelligence committee who has pledged to press Brennan on drones, said in television interviews on Thursday he was encouraged by Obama's decision to provide classified documents, but that more action is needed.

“To make very clear: I am going to push for more declassification of these key kinds of programs, and I think we can do that consistent with national security,” the Oregon Democrat told MSNBC. Asked on NBC if he would still block Brennan's nomination, Wyden declined to give his position.

Carney said Obama supports public discussion of the drone program, and that he has spoken about it publicly and would again.

“The president believes these are weighty matters and questions about how we move forward in our counterterrorism efforts are so important and the need to build a legal structure that guides those efforts, that survives in place beyond this administration,” he said.

The White House's reluctance to release the classified information had angered lawmakers, including Democrats like Senator Dianne Feinstein, who heads the Senate Intelligence Committee.

Feinstein said she was pleased by Obama's decision to release the documents. “It is critical for the committee's oversight function to fully understand the legal basis for all intelligence and counterterrorism operations,” she said.

Republicans praise the drone program.

“The drone program to me is a logical use of how you deal with an enemy combatant,” Senator Lindsey Graham told reporters on Wednesday.

Graham, one of the Republican senators most vocally opposed to Hagel's appointment, said he is “totally supportive” of the administration's rationale for using drones.

Brennan first surfaced as an Obama CIA nominee in 2008. He withdrew after human rights activists protested against his public statements about the agency's use of what it calls “enhanced interrogation” techniques, including the simulated drowning practice known as waterboarding, which a wide range of authorities regard as torture. - Reuters