WASHINGTON - President Barack Obama's nominee to head the Central Intelligence Agency, John Brennan, is expected to face tough questioning about leaks of sensitive information and US spy activities from waterboarding to the use of drones when he appears at a Senate confirmation hearing on Thursday.

In a written submission posted on the Senate Intelligence Committee's website on Wednesday, Brennan acknowledged for the first time that he had given voluntary interviews in connection with investigations into leaks that are being conducted by federal prosecutors in Baltimore and Washington.

John Brennan, nominee for CIA Director, arrives at a meeting with Senate Intelligence Committee Chairman Dianne Feinstein (D-CA) on Capitol Hill in Washington, in this Jan 31, 2013. [Photo/Agencies]

Brennan said the investigations related to cyber warfare against Iran and a foiled bomb plot tied to al-Qaida's Yemen-based affiliate. Brennan said his lawyer had been told by prosecutors that he was "only a witness" in both investigations.

However, leaks are only one of the major issues about which intelligence committee members plan to question Brennan, a former CIA executive under President George W. Bush, who has become the steward of Obama's drone policies.

Congressional aides said much of the questioning is expected to focus on what Brennan knew about the Bush administration's use of harsh interrogation techniques on Islamic militant suspects captured and held, sometimes in secret CIA prisons, after the Sept 11, 2001 attacks on the United States.

Human rights activists and many US politicians, including intelligence committee chairwoman Senator Dianne Feinstein and Republican Senator John McCain, have condemned some of the interrogation techniques as torture.

Democrats also are expected to question Brennan, the top White House adviser on counterterrorism and homeland security, on the Obama administration's use of armed drone aircraft to attack suspected al-Qaida militants and encampments in countries including Afghanistan, Pakistan and Yemen.

Despite the range of expected questions, Senate aides and political handicappers say they have not sensed a groundswell of opposition to Brennan's nomination.

Brennan first surfaced as an Obama CIA nominee in 2008 but he withdrew after human rights activists protested his equivocal public statements regarding the agency's use of "enhanced interrogation" techniques, including the simulated drowning practice known as "waterboarding".

This time human rights groups have raised questions about Brennan, but opposition has come nowhere close to the level mounted by conservative and pro-Israel groups against Chuck Hagel, Obama's nominee to replace Leon Panetta at the Pentagon.

Waterboarding questions

Brennan's knowledge of waterboarding and other harsh interrogation techniques is likely to provoke intense committee questioning.

As Reuters reported earlier this month based on sources, classified CIA message traffic shows that Brennan had detailed, contemporaneous knowledge of the CIA's use of "enhanced interrogation techniques", including waterboarding.

After he temporarily left government service in 2005, Brennan publicly renounced waterboarding and other physically painful interrogation techniques. He and Obama administration officials have also said that Brennan had objected privately to fellow intelligence officers about the techniques' use.

Former Bush administration officials involved in the program say they do not recall such objections.