US President Donald Trump has condemned the "horrific massacre" of mosques in New Zealand that killed at least 49 people and wounded dozens during Friday prayers.

"I express my warmest sympathy and best wishes to the New Zealanders after the horrific massacre of the mosques," Trump said via Twitter. "We stand next to New Zealand after the horrific massacre in the mosques."

A moment ago, White House spokeswoman Sarah Sanders said, "We are in solidarity with New Zealanders and their government against this evil and hate act" in Christchurch.