Media playback is unsupported on your device Media caption Donald Trump tells ABC News that "torture works"

President Trump has indicated that he is considering a return to the sort of harsh interrogation techniques of "enemy combatants" that have been widely condemned as torture, as well as a return to so-called CIA "black sites".

In his first interview since becoming US President, Mr Trump said intelligence officials had told him that "torture absolutely works", but that he would defer to advice from his new CIA director and his secretary of defence. The latter, retired Marine Corps officer Gen James Mattis, says torture does not work.

So what are the global implications if the president goes ahead, asks BBC security correspondent Frank Gardner.

There is a South African proverb, dating from the apartheid era, that goes like this: "How do you catch an elephant? You catch a mouse and keep beating it up until it admits it's really an elephant."

Ridiculous as this may sound, there is an echo of truth here. Torture hurts. That's the whole point of it.

So if someone is tortured badly enough they will say anything to make it stop, including making things up that they think their tormentors will want to hear.

Prisons in certain Middle Eastern countries, especially Syria, are crammed full of people who are being abused so badly they will eventually sign any "confession" to make the treatment stop. In some countries forced confessions remain to this day the primary tool in the prosecutor's armoury.

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In the immediate aftermath of the 9/11 attacks in 2001 the US intelligence community, having failed to prevent the worst attack on the US since Pearl Harbor, became convinced that a second catastrophic attack was on its way.

As President George W Bush's "war on terror" got underway, the normal safeguards of respect for human rights and the rule of law were cast aside in a desperate hunt to find "the ticking bomb".

Top al-Qaeda planners like Ramzi Bin Al-Shibh, Abu Zubaydah and Khaled Sheikh Mohammed, all caught in Pakistan, were "rendered" (transported) to so-called "black sites" for extreme interrogation. These were secret, unacknowledged prisons, run by the CIA and scattered around the globe in Afghanistan, Thailand, Poland, Romania and other countries.

There they were subjected to repeated waterboarding, which makes the bound and helpless victim feel like they are drowning. Khaled Sheikh Mohammed was waterboarded a staggering number of times, well over 100.

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And yet years later, when in 2014 the US Senate's Intelligence and Security Select Committee issued its report on the use of torture under the Bush administration it concluded that torture was "not an effective means of acquiring intelligence or gaining cooperation from detainees".

On Thursday, the US House Speaker, Paul Ryan, said torture was not legal and that the committee agreed it was not legal. Senator John McCain, who chairs the Senate Armed Services Committee, also opposes it.

"The president can sign whatever executive order he likes," he said, "but the law is the law. We are not bringing back torture in the USA."

Morally wrong

There would be strong resistance too from both America's allies and from within the intelligence community itself.

There is a general acceptance now, in most of the world, that those practices carried out in the early years after the 9/11 attacks - extraordinary rendition, detention without trial, enhanced interrogation - were not only morally wrong, they were also counter-productive.

They very rarely produced useful, actionable intelligence. They traumatised not only the victims, some of whom were completely innocent, but also those who witnessed the shocking dehumanising of an individual. Undoubtedly this has given the green light to some unscrupulous practices by regimes who see America's earlier use of torture as a license to do what they like to their own citizens.

Unthinkable as it sounds now, the US even rendered one "high value detainee" to his own country - Syria - for interrogation, knowing that there would be few restraints on his treatment there.

Media playback is unsupported on your device Media caption John McCain said he'd have Donald Trump in court in 'a New York minute' if he reinstated waterboarding

There is also the legal aspect. In 2010 David Cameron, who was then UK prime minister, set up a judge-led, independent inquiry into allegations of complicity by MI5 and MI6 officers in torture.

Career intelligence officers who had thought they were doing the right thing at the time - such as, hypothetically, being within earshot of the harsh interrogation of a suspect in a Pakistani jail - found themselves being questioned by detectives from the Metropolitan Police.

The inquiry was eventually scrapped but it has at least led to a widespread rethink on respect for human rights inside intelligence agencies on both sides of the Atlantic.

Senior intelligence officers who lived through this difficult period are likely to strongly resist turning the clock back and returning to those days.

It is also questionable whether the US would find willing partners to host black site prisons amongst those countries only too relieved to have closed that chapter in their national histories.