US spymaster John Brennan firmly defended CIA officers on Thursday as "patriots" but admitted some interrogators had used "abhorrent" tactics in the past decade.

US spymaster John Brennan firmly defended CIA officers on Thursday as "patriots" but admitted some interrogators had used "abhorrent" tactics in the past decade.

In an extraordinary news conference, broadcast live from the agency's Langley headquarters in a first in CIA history, Brennan sought to play down a Senate report on CIA torture of Islamist suspects that sparked a global furor.

Brennan insisted the vast majority of CIA officers “performed admirably” but he confirmed some had strayed "outside of bounds" of approved rules and abused prisoners.

Brennan said the torture came amid fear of another wave of violence from Al-Qaeda after the September 11, 2001 attacks.

"We were not prepared," he said, describing how then-president George W. Bush had approved the so-called "enhanced interrogation techniques" now denounced as torture.

President Barack Obama halted the program upon taking office in 2009 and has since described the Bush-era use of torture by the CIA as “counterproductive” and an “affront to American values.”

"In a limited number of cases, agency officers used interrogation techniques that had not been authorized, were abhorrent and rightly should be repudiated by all," Brennan said.

Brennan said it was "unknowable" whether harsh interrogations had won useful intelligence.

When asked about his public condemnation of the methods five years ago, Brennan said he stood by his remarks and that torture often produced unreliable intelligence.

"I tend to believe that the use of coercive methods has a strong prospect for resulting in false information," he said.

Brennan said answers from detainees were indeed useful in the hunt for Osama bin Laden, but it was impossible to say whether the "enhanced" interrogation had been necessary.

"There's no way to know if information obtained from an individual who had been subjected at some point during his confinement could have been obtained through other means," he said.

Brennan refused to say whether the methods amounted to torture, but said the CIA was no longer involved in interrogating suspects and has adopted reforms to prevent such abuses from happening again.