US Senator Cory Booker, who is also seeking the Democratic nomination, called on Biden to apologise.

"Frankly, I'm disappointed that he hasn't issued an immediate apology for the pain his words are dredging up for many Americans. He should," Booker, who is black, said in a statement on Wednesday.

The criticism exposed bubbling racial and generational tensions within the Democratic field that is the most diverse in history. Biden, 76, is leading in early opinion polls in the crowded Democratic contest to take on Republican President Donald Trump in the November 2020 election.

Speaking to reporters on Wednesday evening, Biden was asked about Booker's demand that he apologise.

"Apologise for what? Cory should apologise. He knows better. Not a racist bone in my body. I've been involved in civil rights my whole career," Biden said.

At a fundraiser in Potomac, Maryland, a suburb of Washington, Biden cited civil rights leader the Reverend Martin Luther King Jr. as a personal "hero" and an inspiration for his political career.

Biden's campaign said he was not endorsing the positions of the segregationists he named but using them as an example of someone with whom he disagreed.

At issue are Biden's remarks at a New York fundraiser for his presidential campaign on Tuesday night.

Biden said US leaders had lost the ability to work together. He pointed to two segregationists from the South who were serving in the Senate when he was first elected - Democratic Senators James Eastland of Mississippi and Herman Talmadge of Georgia.

"At least there was some civility. We got things done. We didn't agree on much of anything. We got things done. We got it finished," Biden said, according to a pool report distributed by his campaign. "But today, you look at the other side and you're the enemy. Not the opposition, the enemy. We don't talk to each other anymore."

Australian Associated Press