The United Nations human rights chief voiced concern on Monday over what he called President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's "persistent flirtation" with the idea of bringing back torture.

"If other leaders start to follow the same rhetorical course, undermining the [U.N. Convention against Torture] with their words, the practice of torture is likely to broaden, and that would be fatal," U.N. High Commissioner for Human Rights Zeid Ra'ad Hussein said during a speech in London, according to Reuters.

He also warned that Americans could be swayed to support torture in the event of an attack in the U.S. and that Trump's rhetoric poses a threat to international law and principles.

"Mindful of how the American public has, over the last 10 years, become far more accepting of torture, the balance could be tipped in favor of its practice," Hussein said.

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"The president of the United States of America has said that torture could be necessary in certain circumstances," he said. "There is no longer any pretense. They are breaking long-held taboos."

Trump has said in the past that he believes that torture works as an interrogation technique and an intelligence-gathering tool and has expressed support for tactics like waterboarding.

But he has also vowed to defer to Defense Secretary James Mattis on whether the U.S. will return to such practices. Mattis opposes the use of torture techniques.

Hussein also slammed British Prime Minister Theresa May Theresa Mary MayAre US-Japan relations on the rocks? Trump insulted UK's May, called Germany's Merkel 'stupid' in calls: report Bolton says Boris Johnson is 'playing Trump like a fiddle' MORE for her promise in the wake of a series of terror attacks in the United Kingdom to roll back human rights laws if they interfere with the government's ability to step up counterterrorism measures.

"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," Hussein said.

— Updated 8:47 a.m.