WASHINGTON (AFP) - A US psychologist who helped develop the CIA's "enhanced interrogation" programme widely considered torture lambasted critics Tuesday and said America must develop new methods of "legal coercion".

The issue of how far America should go in interrogating suspects gained new urgency when President-elect Donald Trump said while campaigning that he would "bring back a hell of a lot worse than waterboarding" for suspected extremists, though he has since dialed back his rhetoric.

James Mitchell, along with his former colleague John "Bruce" Jessen, worked under CIA contracts worth millions of dollars to advise the spy agency on waterboarding and other techniques beginning in 2002.

Speaking at a Washington think tank, Mitchell insisted the information gleaned by waterboarding several suspects including Khalid Sheikh Mohammed - the self-proclaimed mastermind of 9/11 - was vital and helped save US lives.

"The American public needs to have a debate about how they want to protect themselves," Mitchell told the American Enterprise Institute.

"They need to ask themselves - and I would ask President-elect Trump this - what are you going to do when you have credible evidence, like the CIA did, of another pending catastrophic attack... and the person you are interrogating isn't responding?"

Mitchell claims to have interrogated 14 of the most "high-value" detainees in US custody, including Mohammed and Abd al-Rahim al-Nashiri, suspected of attacking the destroyer USS Cole in the port of Aden that killed 17 US military personnel in 2000.

Both men are being kept indefinitely at the US military prison in Guantanamo Bay pending trials that likely won't occur for years.

Mitchell said he had spent thousands of hours interviewing Al-Qaeda detainees and described Mohammed as a "brilliant" but evil mind who had worked out the intricate details of numerous new plots.

The psychologist recounted Mohammed telling him how he had beheaded Daniel Pearl, a Wall Street Journal reporter who was kidnapped in 2002.

Mitchell said he or Jessen asked Mohammed if "it was difficult for you to do," from an emotional standpoint.

"He said: 'Oh no, I had sharp knives, the toughest part was getting through the neck bone,'" Mitchell said.

Since his election, Trump appears to have modified his views on waterboarding - a change that may reflect the influence of his nominee to head the Pentagon, retired Marine general James Mattis.

In an interview with The New York Times, Trump recounted how Mattis had said that winning a prisoner's trust is a far more effective way of prying information.

"'Give me a pack of cigarettes and a couple of beers and I'll do better,'" Trump said Mattis had told him.

Mitchell suggested Trump had taken the general's quote out of context.

"We know that (rapport-building approach) didn't work for KSM because he had two days of conversation and tea. All he did was rock and pray and taunt the interrogators," the psychologist said, referring to Mohammed.

The United States must develop "some form of legal coercion to move (suspects) along so you can start using social influence to get them talking," he added.

While indicating he did not want to be the "poster boy" for waterboarding, Mitchell blasted America's "obsessive political correctness." If it continues, "we are going to be standing on the moral high ground, looking down into a smoking hole that used to be several blocks in Los Angeles," he said.

"At some point, someone is going to have to make some hard decisions." The CIA's use of torture was detailed in an explosive Senate report released two years ago that describes the agency's interrogation of Al-Qaeda suspects, including waterboarding, beatings, "rectal rehydration" and sleep deprivation.

The American Psychological Association, from which Mitchell resigned in 2006, said that, if the interrogation techniques detailed in the report were truly used, Mitchell and Jessen should be held "fully accountable for violations of human rights."

"Those people are not part of my life. I don't care what they think," Mitchell said of an APA report condemning psychologists who worked with the CIA.

Sarah Dougherty, a senior fellow at Physicians for Human Rights, later said Mitchell had glossed over his role in waterboarding.

"Health professionals should play no role in things like waterboarding, which are clearly torture," she told AFP.

Two former detainees and the estate of another captive who died in custody are suing Mitchell and Jessen over their "barbaric" methods.

Mitchell's appearance came as he is publicizing his book, "Enhanced Interrogation: Inside the Minds and Motives of the Islamic Terrorists Trying To Destroy America."