Donald Trump needs to acknowledge that the extreme interrogation technique known as waterboarding is torture and therefore absolutely prohibited, the U.N. special rapporteur on torture has said.

“Without any doubt, waterboarding amounts to torture,” Nils Melzer said in a statement, adding that a U.S. revival of torture would be catastrophic because other countries would also "get back into the torture business".

“I urgently appeal to President Trump to carefully consider not only U.S. legal obligations, doctrine and tradition, but also the consolidated legal and moral views of the entire international community before allowing the re-introduction of methods or interrogation that are more closely associated with barbarism than with civilisation," he said.

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Waterboarding, in which a victim is subjected to simulated drowning to force them to confess or divulge information, was used by the CIA on three suspected militants detained in secret foreign prisons during President George W. Bush's administration.

But the technique was denounced as torture by President Barack Obama, other top officials, lawmakers and human rights groups. The U.S. Senate also voted to ban it in 2015.

Trump said last week, however, he feels waterboarding works but he will defer to Defence Secretary James Mattis, who does not "necessarily believe" such techniques work.