The US President Donald Trump again invited foreign interference in a US presidential election by publicly calling on China to investigate Democratic political rival Joe Biden, the kind of request that has already triggered an impeachment inquiry in Congress.

The Republican president said he believed both China and Ukraine should look into the 2020 presidential hopeful and his businessman son Hunter, and described the impeachment probe as "crap".

"And by the way, likewise, China should start an investigation into the Bidens. Because what happened in China is just about as bad as what happened with Ukraine," Mr Trump told reporters at the White House.

Mr Trump and his personal lawyer Rudy Giuliani have offered no evidence for their assertions of corruption against former Vice President Biden, a leading contender for the Democratic Party nomination to run against Mr Trump.

Questioned about whether he had asked Chinese President Xi Jinping to investigate, Mr Trump said: "I haven't, but it's certainly something we can start thinking about."

The Chinese Embassy in Washington did not immediately respond to a request for comment.

Mr Trump's appeal to China was particularly striking given that the two countries are locked in a bitter trade war which has damaged global economic growth.

They are due to hold another round of talks in the United States next week.

The China remarks stiffened the resolve of Democrats in the House of Representatives to push ahead with a probe into whether Mr Trump should be impeached following revelations by an anonymous whistleblower who said that Mr Trump asked Ukraine in July to investigate the Bidens.

Hunter Biden was on the board of a Ukrainian gas company at a time when the United States and Europe were trying to woo Ukraine away from next-door-neighbour Russia.

Some Republicans have criticised Mr Biden for taking his son on the Air Force Two vice presidential plane in 2013 to China, where Hunter had business interests.

At a healthcare event in The Villages, Florida, Mr Trump said Democrats had launched the impeachment probe because they cannot beat him at the ballot box in 2020.

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"That's why they do the impeachment crap - they know they can't win," Mr Trump said.

While the election is still a long way off, Reuters/Ipsos polling shows Mr Trump narrowly trailing the top Democratic candidates, including Biden, in hypothetical matchups.

The impeachment probe could lead to the approval of articles of impeachment - or formal charges – against Mr Trump in the Democratic-controlled House.

A trial on whether to remove him from office would then be held in the Republican-controlled Senate.

Many Republicans, however, have shown little appetite for ousting him.

House Republican leader Kevin McCarthy wrote to Democratic House Speaker Nancy Pelosi yesterday asking that all efforts surrounding the impeachment inquiry be suspended until "customary rules and procedures are established".

He asked whether she planned to hold a vote of the full House authorising the inquiry, and whether the president's counsel would be allowed to attend all hearings and depositions.

Refusing to do these things would contradict previous impeachment procedures, Mr McCarthy said.

The White House plans to send Ms Pelosi a letter, possibly today, that will argue Mr Trump and his team can ignore politicians' demands until she holds a full House vote formally approving an impeachment inquiry, a source familiar with the plan said.

Ms Pelosi announced the impeachment investigation last week after the whistleblower lodged a complaint about Mr Trump asking Ukrainian President Volodymyr Zelenskiy to look into investigating the Bidens.

Mr Trump froze $400 million in US aid to Ukraine shortly before speaking to Mr Zelenskiy, prompting accusations from Democrats that he had misused US foreign policy for personal gain.