US President Donald Trump has said repeatedly he believes strongly that "enhanced interrogation techniques" will provide intelligence services an edge when protecting America from its enemies.

While his opponents and human rights groups have called the techniques illegal torture, Tump has said he believes the US intelligence services need to "fight fire with fire".

"When they're chopping off the heads of our people, and other people. When they're chopping off the heads of people because they happen to be a Christian in the Middle East – when Isis is doing things that nobody has ever heard of since medieval times, would I feel strongly about waterboarding? As far as I'm concerned, we have to fight fire with fire," the president said in an interview with ABC.

Trump faces opposition in his move to bring back waterboarding. If Trump were to reinstate the technique, he would be violating a 2015 US law and go against the view of his Defence Secretary James Mattis.

CIA director, Mike Pompeo, told senators earlier in January that he would not sanction the use of torture, but later said he would consider reinstating waterboarding under certain circumstances.

However, the president said he has also spoken to "people at the highest level of intelligence" who told him that waterboarding and torture "absolutely" works. So how does waterboarding work?

In most waterboarding techniques a prisoner is tied to a board with their head raised slightly above their feet. A material such as cloth or cellophane is wrapped over the prisoner's face and water is poured over the material.

Demonstrators simulate waterboarding at the Justice Department in Washington (Reuters) Reuters

The effect of the flowing water simulates the effect of drowning so that in essence the subject feels like they will die. As the gag reflex sets in, coupled with the need to inhale and exhale the prisoner is confronted with the immediate fear of death and will instinctively yield.

ABC reports that CIA officers who employed the waterboarding technique after it was sanctioned for use during the 2003 invasion of Iraq said prisoners lasted on average 14 seconds before they gave up.

Because of the efficacy of the waterboarding technique in making prisoners fear they are going to die, CIA operatives have questioned its usefulness in extracting confessions.

It is "bad interrogation. I mean you can get anyone to confess to anything if the torture's bad enough," CIA officer Bob Baer was quoted as saying.

John Sifton from Human Rights Watch said in 2007 while the technique was being used that it was essentially a violation of international law. "The person believes they are being killed, and as such, it really amounts to a mock execution, which is illegal under international law," he said.