Washington DC - Donald Trump will refuse to accept the results if he loses the US election this year, President Joe Biden said Wednesday.

"He may not accept the outcome of the election," Biden told CNN in an interview. "I promise you he won't, which is dangerous."

The 81-year-old, who is already the oldest person ever to hold presidential office in the United States, said US allies were hoping he would win a second term in November.

"Look, I travel around the world; other world leaders, you know what they all say? Not a joke.

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"80%, after we have a major meeting [say] 'You gotta win... my democracy's at stake.'"

Biden said comments Trump has made on the campaign trail demonstrate how he will bend the presidency to his own ends if he wins back the White House.

"He's saying... he's gonna make sure his attorney general prosecutes those he tells him to prosecute, and if he doesn't, he's going to fire him.

"He said this about prosecution across the board. 'I'm your retribution.' What president has ever said anything like this stuff? But he means it."

At a recent rally in Wisconsin, Trump again sowed doubt by saying he would only accept the election results "if everything's honest."