Now Donald Trump wants China to investigate former vice president Joe Biden

President Donald Trump has called on another nation to probe former vice president Joe Biden.

"China should start an investigation into the Bidens," said Mr Trump in remarks to reporters outside the White House.

Mr Trump, who is the target of an impeachment investigation after making such a request to Ukraine, said he had not directly asked Chinese President Xi Jinping to investigate Mr Biden and his son Hunter but said it's "certainly something we could start thinking about".

Mr Trump and personal lawyer Rudy Giuliani have also tried to raise suspicions about Hunter Biden's business dealings in China, leaning on the writings of conservative author Peter Schweizer.

But there is no evidence that the former vice president benefited financially from his son's business relationships.

Mr Trump's requests for Ukrainian President Volodymyr Zelenskiy to dig up dirt on Mr Biden, as well as Mr Giuliani's conduct, are at the centre of an intelligence community whistleblower complaint that sparked the House Democratic impeachment probe last week.

Mr Trump's comments came as he publicly acknowledged that his message to Mr Zelenskiy and other officials was to investigate the 2020 Democratic presidential contender.

Mr Trump's accusations of impropriety are unsupported by evidence.

"It's a very simple answer," Mr Trump said of his call with Mr Zelenskiy.

"They should investigate the Bidens."

Mr Trump has sought to implicate Mr Biden and his son in the kind of corruption that has long plagued Ukraine.

Former US vice president Joe Biden (Niall Carson/PA)

Hunter Biden served on the board of a Ukrainian petrol company at the same time his father was leading the Obama administration's diplomatic dealings with Kiev.

Though the timing raised concerns among anti-corruption advocates, there has been no evidence of wrongdoing by either the former vice president or his son.

Mr Biden is currently one of the pacesetters in the Democratic Party primaries that will determine Mr Trump's opponent in next year's presidential election.