Prime Minister Jacinda Ardern has requested "sympathy and love for all Muslim communities" from the United States, after being asked by President Donald Trump what assistance they could provide.

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Various world leaders including Mr Trump called the Prime Minister after the Christchurch terror attack at two mosques left 49 dead.

"He very much wished for his condolences to be passed on to New Zealand," Ms Ardern said from the Beehive in Wellington this morning.

Mr Trump then asked Ms Ardern what offer of support the US could provide.

"My message was sympathy and love for all Muslim communities," Ms Ardern said.

"I simply conveyed the sentiment that exists here in New Zealand."

She said Mr Trump "acknowledged that and agreed".

According to the Washington Post, Mr Trump today was asked at the White House if white nationalism was a growing threat.

"I don't really," he answered. "I think it's a small group of people that have very, very serious problems."

"I guess if you look at what happened in New Zealand, perhaps that[s] the case. I don't know enough about it yet," the Washington Post reported.

When asked if she agreed with the view that white supremacy world wide was not a growing problem, Ms Ardern said, "no".