(CNN) US President Donald Trump ignited a row over the use of waterboarding Wednesday after claiming intelligence professionals told him it "absolutely works."

In an interview with ABC News, he said the US must "fight fire with fire" when dealing with terrorists in comments which reverberated around the world.

And while he did concede he would follow the lead of his Secretary of Defense James Mattis and his CIA director Mike Pompeo, Trump's remarks started a worldwide debate over the use of Torture.

What did Trump say?

The Senate voted overwhelming to ban torture across the US government in 2015, codifying a ban President Barack Obama issued by executive order shortly after he was sworn in in 2009. Obama then signed the updated defense authorization bill into law -- but Trump has suggested he's not against reversing such a position.

He says that the US is "not allowed to do anything" while ISIS posts video online of executions carried out by beheading.

Trump ABC interview

"I've spoken as recently as twenty-four hours ago, with people at the highest level of intelligence, and I asked them the question," Trump said.

"Does it work? Does torture work? And the answer was yes. Absolutely.

Trump also stated if Pompeo and Mattis did not want to go down the route of Torture then "that's fine."

He added: "If they do want to do it, I will work toward that end. I want to do everything within the bounds of what you're allowed to do legally. But do I feel it works? Absolutely I feel it works."

Does torture work?

In 2014, the US Senate Select Committee published a report on the CIA's detention and interrogation program, offering a scathing analysis.

The report said that the "use of the CIA's enhanced interrogation techniques was not an effective means of obtaining accurate information or gaining detainee cooperation."

Torture According to the United Nations Convention against Torture, the term "torture" is defined as follows. "Any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted on a person for such purposes as obtaining from him or a third person information or a confession, punishing him for an act he or a third person has committed or is suspected of having committed, or intimidating or coercing him or a third person, or for any reason based on discrimination of any kind, when such pain or suffering is inflicted by or at the instigation of or with the consent or acquiescence of a public official or other person acting in an official capacity. It does not include pain or suffering arising only from, inherent in or incidental to lawful sanctions." According to Amnesty International, 156 countries have signed the UN Convention against Torture.

The 525-report, a brief summary of the 6,700 page document, was the result of a five-year investigation into detention and "enhanced interrogation techniques" used by the CIA against suspected terrorists in secret sites around the world.

It condemned the tactics as "deeply flawed" and often resulting to "fabricated information."

"Anyone who understands interrogation knows that more than 99% of interrogations are very successful even without using enhanced techniques," former US army counterintelligence special agent CJ Grisham, who served in Iraq and Afghanistan, told CNN.

"The argument is that we're talking about these hardcore terrorists who are killing people. Hardcore terrorists aren't the ones on which torture works on, they'd rather die anyway.

"The only time these torture techniques work is when you catch a low level guy is mentally feeble and it works because he doesn't think it's worth it."

Professor Shane O'Mara, author of "Why Torture Doesn't Work: The Neuroscience of Interrogation," said Trump's remarks had left him pondering the source of the President's advice.

"With Trump it's very difficult to know who he has been talking to," O'Mara told CNN.

"It's clear by looking at the 2004 Office of the Inspector General of the CIA report or the CIA emails in the Senate Torture Report that torture really was an institutional disaster for the CIA and hard to believe anyone there would want to go back to it."

America's use of torture

The September 11 attacks forced the US security services to radically alter their methods. Six days after the attacks, President George W. Bush signed a secret memo enabling the CIA to detain suspected terrorists. Then in February 2002, he issued an executive order which declared "members of al Qaeda, the Taliban, and associated forces [as] unlawful enemy combatants who are not entitled to the protections that the Third Geneva Convention provides to prisoners of war."

Thus the stage was set for the adoption of what euphemistically became known as "enhanced interrogation" at secret CIA prisons around the world, known as "black sites".

In 2004, details emerged of the abuses committed at Abu Ghraib, and subsequently the waterboarding of prisoners at Guantanamo Bay.

But it was not until 2014 with the publication of the Senate report that a full picture emerged of the program of systematic torture undertaken by the US between after 9/11.

According to the Senate report , 119 detainees were held at CIA states between 2002 and 2008 -- 39 of those were subjected to so-called enhanced interrogation techniques, ranging from sleep deprivation, to waterboarding, prolonged standing, and exposure to cold.

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At least three men were waterboarded and other psychological tactics involved keeping detainees in pitch-black rooms "with loud noise or music and only a bucket to use for human waste."

At least five detainees were subjected to rectal rehydration -- the technique of force-feeding pureed food by a tube inserted in the rectum.

One detainee, "who had been held partially nude and chained to a concrete floor" died in November 2002 from suspected hypothermia.

The waterboardings of 9/11 mastermind Khalid Sheikh Mohammed became, according to the report, "a series of near-drownings." He was waterboarded at least 183 times

The report drew an angry response from the CIA, which said it contained "too many flaws" for to "stand as official record of the program." It added that many of the charges were "based on authors' flawed analysis of the value of the intelligence obtained from the detainees."

Will Trump bring back torture?

Trump's CIA director has said that he would not restart the CIA's use of enhanced interrogation tactics the that fall outside of army field manual, even if requested by the President.

When asked about such a scenario at his confirmation hearing, Pompeo said: "Absolutely not. Moreover, I can't imagine I would be asked that by the President-elect."

John McCain, Chairman of the Senate Armed Services Committee, who was tortured as a prisoner of war in Vietnam, noted that torture is now illegal in the US.

"The President can sign whatever executive orders he likes," McCain said in a statement. "But the law is the law. We are not bringing back torture in the United States of America."

Which other countries use torture?

Torture has been illegal since the Universal Declaration of Human Rights in 1948 while 156 countries have signed the UN Convention against Torture.

But according to Amnesty International's 2015/16 annual report , more than 122 states tortured or otherwise ill-treated people.

It has found instances of torture ranging from South America to Africa, Europe, the Middle East and Asia.

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Amnesty names Philippines and Mexico as places where torture is "widespread and routine in police stations".

It adds that in Morocco and Uzbekistan, "the courts often rely on confessions people have made while being tortured."

Amnesty also cites Nigeria, where "beatings and mock executions are just some of the treatments people face in detention."