US President Donald Trump said "boy, that is just not good" when he was shown a stone block on which slaves were sold, Martin Luther King's niece has claimed.

Alveda King described Mr Trump as appearing "visibly moved" at two moments during his tour of the National Museum of African American History and Culture.

She told The Atlanta Journal-Constitution she heard Mr Trump say "Boy, that is just not good. That is not good," when he saw a stone auction block from Hagerstown, Maryland.

When they later came across a set of shackles used to restrain children, Mr Trump reportedly said: "That is really bad. That is really bad."

Donald Trump's Black History Month speech

The US president was making his first visit to the new Smithsonian along with Ms King, Ben Carson, Senator Tom Scott and his daughter Ivanka.

After the tour, Mr Trump said: “Today and every day of my presidency, I pledge to do everything I can to continue that promise of freedom for African-Americans, and every American. Nothing is more important.”

“This tour was a meaningful reminder of why we have to fight bigotry, intolerance and hatred in all of its very ugly forms."

Ms King told the paper his visit sent a positive message to black Americans.

In pictures: Martin Luther King Jr's campaign for civil rights Show all 10 1 /10 In pictures: Martin Luther King Jr's campaign for civil rights In pictures: Martin Luther King Jr's campaign for civil rights 4510050.jpg Memories of a March and a Dream: Martin Luther King during the March on Washington, on 28 August 1963 AFP/Getty Images In pictures: Martin Luther King Jr's campaign for civil rights Martin Luther King and the March on Washington BBC In pictures: Martin Luther King Jr's campaign for civil rights martin-luther-king-6.jpg American president John F. Kennedy in the White House with leaders of the civil rights 'March on Washington' (left to right) Whitney Young, Dr Martin Luther King Getty Images In pictures: Martin Luther King Jr's campaign for civil rights martin-luther-king-7.jpg American civil rights campaigner Martin Luther King (1929 - 1968) arriving at London Airport. He is in England to be the chief speaker at a public meeting about colour prejudice and to appear on the BBC television programme 'Face To Face' Getty Images In pictures: Martin Luther King Jr's campaign for civil rights georgiamlk.jpg Martin Luther King Jr In pictures: Martin Luther King Jr's campaign for civil rights Picture: Library of Congress/ Wiki Commons In pictures: Martin Luther King Jr's campaign for civil rights martin-luther-king-8.jpg Civil rights Leaders hold hands as they lead a crowd of hundreds of thousands at the March on Washington for Jobs and Freedom, Washington DC Getty Images In pictures: Martin Luther King Jr's campaign for civil rights 08peoobi1.jpeg Height, far right, listens to Martin Luther King's 'I have a dream' speech in Washington in 1963 AP In pictures: Martin Luther King Jr's campaign for civil rights 4260367.jpg Beyond the 'us' and 'them' mentality: Martin Luther King's 'I Have a Dream' speech in Washington DC in 1963 AP In pictures: Martin Luther King Jr's campaign for civil rights v2-MLKAFP.jpg Martin Luther King had been imprisoned for taking his campaign of non-violent protest to the streets of Birmingham, Alabama AFP/Getty Images

However, Janice Mathis, executive director of the National Council of Negro Women, said: “History is always instructive and the museum tells a powerful story, so it is unlikely that he wouldn’t be moved by his visit.

“Now, it is my hope that the visit will move beyond a celebration of Black History Month and that he will now consider public policy that is appropriate for a culturally and racially diverse nation today.”