President Donald Trump referred to the Christchurch mosque attacks in a speech at the White House in honour of the upcoming National Day of Prayer.

The president told those gathered that people of faith around the world have faced terrible hardship of late.

Trump says violence and terrorism against people of all faiths must end, and that "all civilised nations must join together in this effort."

Trump spoke to faith leaders at the National Day of Prayer dinner, an event that takes place tomorrow.

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"America will be a nation that believes forever, and we certainly believe – more than anyone – the power of prayer," Trump declared in the State Dining Room before continuing with an unscripted flourish: "It's the most powerful thing there is."

Trump paid respects to Jewish-Americans killed and wounded last week at the Chabad of Poway synagogue in California. He also recalled the attacks on Christians in Sri Lanka last Easter and on Muslims in New Zealand the month before.

He also cited the burning of three black churches in Louisiana and last year's shooting spree at a Pittsburgh synagogue.

"All of us in this room send our love and prayers to the Jewish Americans wounded at the Chabad of Poway shooting in California," he said. "And our hearts break for the life of Laurie Gilbert-Kaye who was so wickedly taken from us."

"We mourn for the Christians murdered in Sri Lanka on Easter Sunday and grieve for the Muslims murdered at their mosques in New Zealand," he added. "Here at home, we also remember the three historically black churches burned recently in Louisiana and the horrific shooting last year at the Tree of Life Synagogue in Pittsburgh."

- AP