US President Barack Obama's nominee as CIA director, John Brennan, faces tough questions about the use of drones to kill US terrorism suspects.

Mr Brennan is appearing before a congressional intelligence committee.

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

The hearing was suspended shortly after Mr Brennan started speaking because of protesters, who began yelling "Torture is always wrong" and "Stop the Drones."

Senator Dianne Feinstein, chairwoman of the Senate Intelligence Committee, ordered the room be cleared and said she wanted people to be respectful in the hearing room.

Although there has been no groundswell of opposition to Mr Brennan's confirmation, he was expected to be examined closely about US spy activities from waterboarding to the use of drones at the committee hearing.

Mr Brennan was expected to win confirmation from both the panel and later the full US Senate.

Mr Brennan told the hearing that he had not tried to stop harsh interrogation techniques such as waterboarding when he was at the spy agency earlier in his career, but had objected to them privately.

"I did not take steps to stop the CIA's use of those techniques. I was not in the chain of command of that program," Mr Brennan said.

"I had expressed my personal objections and views to some agency colleagues about ... waterboarding, nudity and others ... but I did not try to stop it, because it was something that was being done in a different part of the agency under the authority of others, and it was something that was directed by the administration at the time."

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 Mr Obama's other nominees, including Chuck Hagel, his choice to lead the Pentagon.

The White House said the release of classified opinion laying out the legal basis for armed drone strikes on US citizens overseas who are suspected in terrorist plots, reflected Mr Obama's understanding of the importance of questions about the drone programme.

Civil liberties groups have criticised the programme as effectively a green light to assassinate Americans without due process in the courts under the US Constitution.

Mr 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.