Donald Trump thatJoe Biden had tried to block a Ukraine corruption probe on his son Hunter.

US President Donald Trump openly called on China as well as Ukraine to investigate his potential 2020 election rival Joe Biden, taunting Democrats seeking his impeachment for inviting foreign election interference.

Speaking in Florida Thursday, President Trump blasted his accusers as "maniacs" pursuing "impeachment crap" as he sought to turn the tables on a probe that threatens to make him only the third US president ever impeached in the House of Representatives, and face a trial in the Senate.

As a former State Department diplomat testified behind closed doors in Congress on his role in the Ukraine scandal, Mr Trump doubled down by calling for Ukraine President Volodymyr Zelensky and Chinese leader Xi Jinping to go after Mr Biden, who leads the race for the Democratic presidential nomination next year.

"I would say that President Zelensky, if it were me, I would recommend that they start an investigation into the Bidens," Mr Trump told reporters outside the White House.

"Likewise, China should start an investigation into the Bidens, because what happened in China is just about as bad as what happened with Ukraine."

Asked if he would request Xi do the same, Mr Trump replied, "It's certainly something we can start thinking about."

Adam Schiff, the Democrat leading the impeachment probe in the House of Representatives, said Mr Trump was acting "with impunity" in the face of the law.

"Once again we have a president of the United States suggesting, urging a foreign country to interfere in our presidential elections," Mr Schiff said.

- Biden rejects allegations -

Mr Trump alleges that Mr Biden, as vice president in 2014, tried to block a Ukraine corruption probe into his son Hunter's business partner, a Ukraine gas tycoon, using US aid as leverage.

He also alleged Hunter used his stature to raise $1.5 billion in 2013 from China for a new investment vehicle in Beijing.

He made "millions" off these investments, Mr Trump alleges.

The record in Ukraine, however shows that there was no corruption probe of Hunter Biden's partner to be blocked, and that the United States and allies all viewed Kiev's since-removed corruption prosecutor at the time as himself deeply compromised.

In China, according to multiple media reports, a business associate of Hunter Biden obtained an investment license that did not include Mr Biden's name, and only raised several million dollars.

Hunter Biden put up $420,000 for a minority interest, and has not received any compensation or return on it, his lawyer told the Washington Post.

Joe Biden's campaign called Mr Trump's comments already-debunked "conspiracy theories" and accused him of "a grotesque choice of lies over truth and self over the country."

"Mr. President, you cannot extort foreign governments to help you win re-election. It's an abuse of power. It violates your oath of office. And it jeopardizes our national security," Mr Biden tweeted separately.

- Ex-Ukraine envoy questioned -

In Congress, former US special envoy to Ukraine Kurt Volker was interviewed by House lawyers for more than eight hours on his role in the efforts by President Trump and the president's personal lawyer Rudy Giuliani to pressure Ukraine to investigate the Bidens.

Text messages released by House Democrats afterward showed Mr Volker telling top Zelensky aide Andrey Yermak that the Ukrainian president should open an investigation sought by Mr Trump in return for a visit to Washington.

"We have ample evidence now that there was a requirement that President Zelensky investigate the 2016 election, and the Bidens, if he wanted to get a meeting," with Mr Trump, said Democrat Eric Swalwell.

Meanwhile, Republicans who sat in on the interview said Mr Volker did not support charges that Mr Trump withheld hundreds of millions of dollars in military aid from Ukraine to force Mr Zelensky to open an investigation, as was alleged in a whistleblower complaint made public last week.

"It is crystal clear... that any allegation that President Trump was trying to get President Zelensky to manufacture dirt on the Bidens is just not true," said Representative Lee Zeldin, before Mr Volker's testimony ended.

Democrats vowed to push through with the impeachment investigation, having subpoenaed documents from Secretary of State Mike Pompeo and Mr Giuliani, and threatened to subpoena the White House.

House Speaker Nancy Pelosi rejected a Republican call to halt the investigation on the grounds that a full House resolution on an impeachment inquiry had not been voted on.

"There is no requirement under the Constitution, under House Rules, or House precedent that the whole House vote before proceeding with an impeachment inquiry," she said.