The director of the CIA insisted Tuesday that US agents' use of brutal interrogation techniques against Al-Qaeda suspects helped prevent attacks, in the wake of a critical Senate report.

John Brennan admitted that mistakes had been made in the years following the September 11, 2001 attacks on US cities.

But he said the Central Intelligence Agency's own review found that harsh interrogations "did produce intelligence that helped thwart attack plans, capture terrorists and save lives."

This is disputed in the Senate report, a redacted version of a review that the CIA had tried to delay, which found that the brutal interrogation program had produced little useful information.

"As noted in CIA's response to the study, we acknowledge that the detention and interrogation program had shortcomings and that the Agency made mistakes," Brennan said, in a statement.

"The most serious problems occurred early on and stemmed from the fact that the agency was unprepared and lacked the core competencies required to carry out an unprecedented, worldwide program of detaining and interrogating suspected Al-Qaeda and affiliated terrorists.

"In carrying out that program, we did not always live up to the high standards that we set for ourselves and that the American people expect of us," he said.

"As an agency, we have learned from these mistakes, which is why my predecessors and I have implemented various remedial measures over the years to address institutional deficiencies."