The most powerful man of the United Nations has paid tribute to the people of Christchurch.

UN Secretary-General António Guterres has been in the garden city, visiting victims of the two mosques targeted by a terrorist on March 15.

In a sit down interview with Newstalk ZB’s Canterbury Mornings host Chris Lynch, he said hate speech is spreading like wildfire on social media and “it needs to be extinguished.”

Every year, the Secretary-General visits a Muslim community during the holy month of Ramadan to express his solidarity.

He said today’s visit in Christchurch was an expression of grief and condolences and more than anything else, an expression of admiration.

“I think the reaction of this community was absolutely wonderful. The forgiveness, the tolerance, the openness they’ve shown, which was matched by the New Zealand people.”

When Lynch asked if he was worried about whether Facebook’s drive to close down white supremacy groups could send them further underground making them more dangerous, António Guterres said “we need to address the root causes of these [groups.]

“It’s not only a question of hate speech in social media, it is a whole strategy.”

In February, the United Nations launched a “global strategy” to combat hate speech, a growing scourge that the Guterres said “poisoned” debate on crucial issues.

Guterres announced the effort, which will be led by his special adviser on genocide prevention Adama Dieng, at the UN Human Rights Council.