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UN Secretary General Antonio Guterres warned hate speech was spreading online "like wildfire" at a meeting with victims of the Christchurch mosque shootings Tuesday, vowing the world body will lead efforts to extinguish the problem.

Guterres visited the Al-Noor mosque, one of two Muslim centres in the New Zealand city where a self-described white supremacist killed 51 people in a March 15 shooting that the attacker live-streamed on Facebook.

The UN chief is travelling the South Pacific to highlight the impact of climate change but said he also wanted to show his support for Christchurch's Muslim community during Ramadan.

"I know there are no words to relieve the hurt and sorrow and pain, but I wanted to come here personally to transmit love, support and total and complete admiration," he said.

He told victims of the worst mass shooting in modern New Zealand history that there had been "a dangerous upsurge in hatred" as social media was exploited to promote bigotry.

"Hate speech is spreading like wildfire in social media. We must extinguish it," the Portuguese diplomat said.

"There is no room for hate speech - online or offline."

He highlighted a previously announced plan for his special adviser on genocide prevention Adama Dieng to combat online extremism.

He said Dieng's mission was to "bring together a United Nations team to scale up our response to hate speech and present a global plan of action".

New Zealand Prime Minister Jacinda Ardern will co-host a meeting of world leaders and tech firms to promote a "Christchurch call" aimed at curbing online extremism.

She has been highly critical of social media giants in the wake of the Christchurch killings, saying they should be "taking ownership and responsibility over their platforms".

Guterres also visited Linwood Mosque where he laid a wreath and met with survivors including Abdul Aziz, who is considered a hero for chasing the gunman and throwing a credit card machine and a discarded gun at him.

Aziz said he was honoured to meet the UN leader.

"To come here and share the pain with us, it means a lot," Aziz said.

Guterres attended a climate alliance event while on his visit to several South Pacific countries primarily to highlight the problems of climate change.

His trip comes ahead of the Climate Action Summit that he plans to convene in September in New York.