Zeid Ra'ad al Hussein also lambasted British Prime Minister Theresa May for threatening to change human rights laws if they got in the way of security operations, saying her words would give heart to authoritarian governments.
The UN High Commissioner for Human Rights told an audience in London late on Monday that strident statements in the wake of attacks risked undermining international rights treaties.
"If other leaders start to follow the same rhetorical course, undermining the (UN Convention against Torture) with their words, the practice of torture is likely to broaden, and that would be fatal.
"The Convention would be scuttled, and a central load bearing pillar of international law removed."
Trump said in late January that he thought the practice of waterboarding a form of simulated drowning worked as an intelligence gathering tool. But he also said he would defer on the issue to Defense Secretary Jim Mattis who disagrees with him about the technique's usefulness.
Zeid said he was worried by Trump's 'persistent flirtation' with a return to torture. There was little immediate danger of the United States using torture, but that could change if there was an attack on US soil, he said.
"Mindful of how the American public has, over the last ten years, become far more accepting of torture, the balance could be tipped in favour of its practice," he said.
After deadly terrorist attacks on London and Manchester, May told The Sun newspaper this month: "If human rights laws get in the way of doing these things, we will change those laws to make sure we can do them."
"Whatever the intention behind her remarks, they were highly regrettable, a gift from a major Western leader to every authoritarian figure around the world who shamelessly violates human rights under the pretext of fighting terrorism," he said.
Zeid also rounded on Philippine President Rodrigo Duterte's rights record police say almost 9,000 people, many small time drug users and dealers, have been killed since he took office.
"The president of the Philippines has spoken openly about extra judicial killings. And the president of the United States of America has said that torture could be necessary in certain circumstances.
There is no longer any pretence. They are breaking long held taboos," Zeid said.
"The dangers to the entire system of international law are therefore very real," he said.
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