Media playback is unsupported on your device Media caption John Brennan: "Unknowable" if techniques revealed useful intelligence

CIA Director John Brennan has defended the agency's post-9/11 interrogation methods but admitted some techniques were "harsh" and "abhorrent".

Speaking at CIA headquarters, he said some officers acted beyond their authority but most did their duty.

A scathing Senate report two days earlier said "brutal" methods like waterboarding were ineffective.

Senator Dianne Feinstein, whose committee produced the report, said torture should now be banned by law.

In his comments Mr Brennan asserted the CIA "did a lot of things right" at a time when there were "no easy answers".

"Our reviews indicate that the detention and interrogation programme produced useful intelligence that helped the United States thwart attack plans, capture terrorists and save lives," Brennan told a rare CIA news conference in Virginia.

But we have not concluded that it was the use of "enhanced interrogation techniques" (EITs) within that programme that allowed us to obtain useful information from detainees who were subjected to them, he added.

"The cause-and-effect relationship between the use of EITs and useful information subsequently provided by the detainee is, in my view, unknowable."

Image copyright Getty Images Image caption The CIA is now in the uncomfortable position of defending itself publicly

While he was speaking, Senator Feinstein was rejecting his arguments on Twitter.

One tweet said: "Brennan: 'unknowable' if we could have gotten the intel other ways. Study shows it IS knowable: CIA had info before torture. #ReadTheReport".

Mr Brennan was a senior CIA official in 2002 when the detention and interrogation programme was put in place.

George W Bush, who was US president at that time, has not commented on the report, but his Vice-President Dick Cheney strongly rejected criticism of the CIA's techniques.

"The men and women of the CIA did exactly what we wanted," he told Fox News.

"We said we've got to go use enhanced techniques … and we're going to find out.

"We've got Khaled Sheikh Mohammed who's the mastermind of 9/11 and he is in our possession, we know he's the architect. And what are we supposed to do? Kiss him on both cheeks and say please tell us what you know? Of course not."

At the scene - Tara McKelvey, BBC News, Langley, Virginia

Image copyright AFP

John Brennan spoke in measured tones and with a deep booming voice in a place that clearly made him uncomfortable - standing at a podium in front of journalists and cameras.

In his speech he tried to show the human side of the CIA. He said that after 9/11 the staff, like others in the US, grieved and prayed.

He said this week was a tough time for people at the agency because of the release of the Senate report. But as he described their situation, he kept his head down and read carefully from the text in front of him. He wanted to make sure he got the words right.

Occasionally he looked up but when he did he gazed at the ceiling as if no-one was in the room. As a result the speech came across as anodyne and bloodless despite the emotionally charged words that were on the page.

An outgoing Democratic Senator, Mark Udall, has called on Mr Brennan to quit, citing interference from the CIA in preparing the report.

The report, a summary of a longer 6,000-page classified report, says that the CIA carried out "brutal" and "ineffective" interrogations of al-Qaeda suspects in the years after the 9/11 attacks on the US and misled other officials about what it was doing.

The information the CIA collected using "enhanced interrogation techniques" failed to secure information that foiled any threats, the report said.

Mr Brennan described the actions of some CIA agents as "harsh" and "abhorrent" but would not say if it constituted torture.

He added an overwhelming number of CIA agents followed legal advice from the justice department that authorised some of the brutal methods.

"They did what they were asked to do in the service of their nation."

Image copyright Getty Images Image caption Fomer US Vice President Dick Cheney said the Senate report was "deeply flawed"

The UN and human rights groups have called for the prosecution of US officials involved in the 2001-2007 programme.

But the chances of prosecuting members of the Bush administration are unlikely - the US justice department has pursued two investigations into mistreatment of detainees and found insufficient evidence.

On Wednesday, an unnamed justice department official told the Los Angeles Times prosecutors had read the report and "did not find any new information" to reopen the investigation.

Key findings:

Image copyright AFP Image caption Khalid Sheikh Mohammed, left, and Abd al-Rashim al-Nashiri, were among those held by the CIA

none of 20 cases of counterterrorism "successes" led to unique or otherwise unavailable intelligence

CIA misled politicians and public

at least 26 of 119 known detainees in custody during the programme wrongfully held

methods included sleep deprivation for up to 180 hours, often standing or in painful positions

Saudi al-Qaeda suspect Abu Zubaydah was kept confined in a coffin-sized box for hours on end

waterboarding and "rectal hydration" were physically harmful to prisoners

What is 'enhanced interrogation'?

Who knew what when?

Who were the detainees?

US President Barack Obama, who stopped the programme in 2009, said some methods amounted to torture.

When asked whether there was a situation where the CIA would use similar interrogations again, Mr Brennan said the CIA was "not contemplating" it, but said he left such decisions up to "future policymakers".