Donald Trump has retweeted three inflammatory videos from a British far-right group.
The first tweet from Jayda Fransen, the deputy leader of Britain First, claims to show a Muslim migrant attacking a man on crutches.
This was followed by two more videos of people Ms Fransen claims to be Muslim.
Responding to Mr Trump's posts, UK Prime Minister Theresa May's official spokesman said it was "wrong for the president to have done this".
White House spokeswoman Sarah Sanders said Mrs May and other world leaders knew that "these are real threats that we have to talk about".
"Whether it's a real video, the threat is real," she said.
"I think it's important to talk about national security and national security threats. The president sees different things to be national security threats, and he sees having strong borders as being one of the things that helps protect people in this country from some real threats that we face."
Britain First was founded in 2011 by former members of the far-right British National Party (BNP).
The group has grabbed attention on social media with controversial posts about what they deem "the Islamification of the UK".
It has put up members to run in European elections and by-elections on anti-immigration and anti-abortion policies, but has not secured any seats.
It also contested the most recent London mayoral election, receiving 1.2% of the vote.
The original video was shared by US conservative commentator Ann Coulter who Mr Trump follows.
Ms Fransen has more than 52,000 followers on Twitter.
She responded enthusiastically to Mr Trump sharing her tweets. She posted on her account: "Donald Trump himself has retweeted these videos and has around 44 million followers!"
"God bless you Trump! God bless America!" she added. The message was also shared on Britain First's Twitter account.