Geneva (AFP) - The UN human rights chief urged Facebook Wednesday to more proactively address hate speech but warned against excessive regulation, after US President Donald Trump accused tech giants' platforms of bias against him.

Zeid Ra'ad Al Hussein's appeal follows a decision by Facebook this week to ban Myanmar's army chief and other top military brass named in a UN probe linking them to a possible "genocide" against Rohingya Muslims.

The social media network is the prime source of news and information for many in the country, but has also been a platform for the army and Buddhist hardliners to spread hate speech against the Rohingya and other minorities.

The site aired support for the military's bloody "clearance operations" last year that forced some 700,000 Rohingya over the border into Bangladesh.

"We felt early on very uncomfortable with what we were seeing in Myanmar, (but) in the early meetings that we had with Facebook, I didn't think they were taking it seriously," said Zeid, who is due to step down from his post as UN High Commissioner for Human Rights at the end of the week.

"Hopefully they've now awoken," he told reporters in Geneva, warning that there could easily be other cases "where Facebook is the dominant medium in a country where you see a deterioration of human rights conditions and then their role would be brought into question."

He cautioned that the company should be "careful", since it risked being subpoenaed in possible future trials "of those accused of the worst crimes."

- 'Accessory' -

"We've seen from the jurisprudence of the past that if you've enabled, you've abetted, you've been an accessory," Zeid said, stressing that while he was not accusing Facebook of that, "they have to be sure that they know where they are and what side of the law they are on."

The Jordanian national, who is set to be succeeded by former Chilean president Michelle Bachelet, urged Facebook to not wait until a full-blown crisis erupts to act against hate speech.

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"They should be thinking proactively about what steps they will take to mitigate that," he said.

Zeid meanwhile warned of going too far in the opposite direction, cautioning that "there is also another danger (of being) overregulated."

He pointed to recent comments by the US president, who has accused big tech firms like Facebook, Google and Twitter of bias against him, slamming what he called "rigged" internet search results.

Trump offered no details on what if any actions he might take, but a top White House aide suggested the administration may look at some type of regulation in response to the president's complaints.

Zeid insisted that instead of more government regulations, the platforms' actions must "be regulated by (international) human rights law."

- 'Dangerous' -

The comments did not mark the first time the UN rights chief has criticised Trump.

He has repeatedly warned that the US president's relentless attacks on the media are "dangerous".

On Wednesday, Zeid said Trump's recent statements "have been very worrisome to me," reiterating his warning that the president's words could trigger violence against journalists.

"All of us would be horrified, but perhaps not shocked if we heard of a journalist that was attacked and severely beaten for example at one of the rallies," he said, stressing that if that happens "the president would bear some responsibility."

"As the campaigning seasons heat up, whether it be for the midterm elections or the presidential elections to come, the conduct of the president should be under very careful scrutiny by the international community and by this office," Zeid said.