Donald Trump slammed the allegations as "ridiculous".

US President Donald Trump and his top challenger in the 2020 election Joe Biden traded accusations on Saturday following reports of a whistleblower's claim alleging the president pressured Ukraine to investigate Biden's son.

On Friday, The Washington Post reported that a whistleblower complaint had been filed over Trump's communications with Ukrainian President Volodymyr Zelensky, and a "promise" allegedly made by the US leader.

Trump allegedly pressured Zelensky to investigate a son of Biden, a former Democratic vice president who is seeking to challenge Trump in the 2020 election.

Biden is leading Trump, a Republican, in most polls.

"This appears to be an overwhelming abuse of power, to get on the phone with a foreign leader who is looking for help from the United States and ask about me," Biden told reporters on the campaign trail in the state of Iowa.

"I know what I'm up against, a serial abuser. That's what this guy is," Biden continued. "If he sees any threat to his staying in power, he'll do whatever he has to do. But this crosses the line."

Biden had on Friday demanded Trump immediately release the transcript of a July 25 call with Zelensky while saying reports that Trump sought to coerce his Ukrainian counterpart, if true, amounted to "clear-cut corruption."

Trump slammed the allegations as "ridiculous," and on Saturday accused Biden on Twitter of having demanded, during his time as vice president, "that the Ukrainian Government fire a prosecutor who was investigating his son."

He also accused news outlets of fabricating "a story about me and a perfectly fine and routine conversation I had with the new President of the Ukraine. Nothing was said that was in any way wrong, but Biden's demand, on the other hand, was a complete and total disaster."

Hunter Biden, Joe Biden's second son, worked for a Ukrainian gas group from 2014, when his father was vice president during Barack Obama's administration.

"I've never spoken to my son about his overseas business dealings," Biden said on Saturday. "Everybody looked at this and everybody looked at it and said there's nothing there."

Later on Saturday, Trump posted a video montage of media reports, which includes a clip of Biden saying he'd asked that the prosecutor be dismissed.

Biden has said the call had nothing to do with his son.