The White House has claimed that Donald Trump was trying to “elevate the conversation” by sharing anti-Muslim videos from a British far-right group.

But the press secretary, Sarah Sanders, acknowledged that she does not believe the US president was aware of the identity of Jayda Fransen, the deputy leader of Britain First who first posted the films.

Trump sparked international uproar on Wednesday when he retweeted three inflammatory videos purporting to show violence being committed by Muslims. Their authenticity has been strongly challenged.

Asked if he was aware of the risk of elevating fringe rightwing groups, Sanders told reporters: “Look, I think what he’s done is elevate the conversation to talk about a real issue and a real threat, and that’s extreme violence and extreme terrorism, something that we know to be very real, and something the president feels strongly about, talking about, and bringing up and making sure is an issue every single day, that we’re looking at the best ways to protect Americans.”

The spokeswoman was challenged over whether Trump knows who Fransen is. “No, I don’t believe so,” she admitted. “But again, I think he knew what the issues are, and that is that we have a real threat of extreme violence terrorism and violence, not just in this country but across the globe, particularly in Europe and that was the point he was making, and I don’t really have much to add beyond that.”

The descriptions of the posts by Fransen read: “VIDEO: Islamist mob pushes teenage boy off roof and beats him to death!” and “VIDEO: Muslim Destroys a Statue of Virgin Mary!” and “VIDEO: Muslim migrant beats up Dutch boy on crutches!”

Questioned over whether Trump watches many such videos, Sanders said only: “I’m not sure of every single video the president has viewed.”

Sanders and other White House officials have staunchly defended the president’s retweeting of the videos and suggested that their veracity is not a high priority. They declined to comment on his tweeted criticism of Theresa May after she said he was “wrong” to have done it.

The press secretary added at Thursday’s press briefing: “I think the president feels that bringing up important issues of our time like extreme violence and terrorism are important to do. That’s what he was doing in that process, and I think he’s going to continue to do that in a number of venues, whether it’s through speeches, whether it’s through Twitter or other social media platforms.”

Sarah Sanders in the White House press room on Thursday. Photograph: Alex Wong/Getty Images

The stance puts Washington directly at odds with its old ally in an extraordinary public rift that prompted an emergency debate in parliament. Politicians ranging from Labour leader Jeremy Corbyn to Nigel Farage, the former head of Ukip and a Trump cheerleader, have condemned the retweets.

Many have called for the invitation of a state visit to be formally withdrawn, and for the government to demand that Trump take down the retweets. On Thursday the White House did not respond to calls, text messages or emails asking whether such a request had been made, or whether Trump would consider it.

Kim Darroch, the British ambassador to the US, and his counterpart Woody Johnson, appointed US ambassador to the UK by Trump, said they had raised concerns about Trump’s retweets with the White House.

Responding to a flood of critical responses to a callout he had started on Twitter, Johnson said: “Thank you for your questions and feedback. I have relayed your concerns to Washington. The US and UK have a long history of speaking frankly with each other, as all close friends do. Our relationship is strong, vital and enduring.”

A typical response to the ambassador’s hashtag was: “I’d like to #askwoody why his president is promoting fascism and hatred in my country and if he thinks this is acceptable.”

Sir Christopher Meyer, the former British ambassador to the US, praised May’s handling of the situation so far. “I think she’s actually judged her reaction to Trump’s extraordinary and horrible behaviour just the right way,” he told BBC Radio 4’s PM programme. “She’s emphasised the importance of the relationship with the United States, as she should have done, and she has publicly rebuked the president for retweeting as he retweeted.

“I can’t remember, in my entire life, let alone my entire career, ever a British prime minister publicly rebuking an American president for misbehavior.”

Sanders also announced that Trump will deliver his first State of the Union address on 30 January next year, and that the president will donate his third quarter salary to the Department of Health and Human Services to assist in the response to the opioid epidemic.