Zeid Ra'ad Al Hussein voiced particular alarm over Trump's verbal assaults on CNN, the New York Times and Washington Post.
"To call these news organisations 'fake' does tremendous damage and to refer to individual journalists in this way, I have to ask the question, is this not an incitement for others to attack journalists?
"And let's assume a journalist is harmed from one of these organisations, does the president not bear responsibility for this, for having fanned this?" Zeid told reporters in Geneva.
"I believe it could amount to incitement," he added, saying Trump had set in motion a cycle that includes "incitement, fear, self-censorship and violence."
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According to the rights chief, Trump's assault on the media has emboldened other countries to crack down on presss freedoms.
"The demonisation of the press is poisonous because it has consequences elsewhere," Zeid said.
He expressed specific concern over Trump's speech in Arizona earlier this month in which journalists were condemned by the US leader as "dishonest people" who "don't like our country".
