U.S. President Donald Trump again invited foreign interference in a U.S. presidential election on Thursday, publicly calling on China as well as Ukraine to investigate Democratic political rival Joe Biden.

The request was similar to the one that has already triggered an impeachment inquiry in Congress, which is now proceeding with a deposition of a former administration official.

The Republican president said he believed both countries should look into the 2020 presidential hopeful and his son Hunter Biden and his business dealings. He also 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," Trump told reporters at the White House.

Trump and his personal lawyer, Rudy Giuliani, have offered no evidence for their assertions of corruption against former vice-president Joe Biden, a leading contender for the Democratic Party nomination to run against Trump, and his son.

Questioned about whether he had asked Chinese President Xi Jinping to investigate, 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.

Trump's appeal to China was particularly striking given that Washington and Beijing are locked in a bitter trade war. They are due to hold another round of talks in the United States next week.

Former Ukraine representative questioned

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

The whistleblower complaint and a summary transcript of the call were released last week. A day later, on Sept. 27, Kurt Volker resigned as special representative for Ukraine negotiations. The complaint says Volker met in Kyiv with Zelensky and other Ukrainian political figures a day after the call, and he provided advice about how to "navigate" Trump's demands.

On Thursday, staff from the House's foreign affairs, intelligence and oversight committees conducted a transcribed interview with Volker. The 54-year-old, also a former U.S. ambassador to NATO, appeared before the committees voluntarily.

Volker told lawmakers he wasn't personally involved in Trump's efforts to have Ukraine investigate the Bidens, adding that he warned the country to steer clear of U.S. politics.

Trump's lawyer, Giuliani, told Reuters on Thursday that Volker never expressed any concerns about his efforts and put him in touch with Andriy Yermak, an adviser to Zelensky.

New York Rep. Lee Zeldin updates reporters outside of a closed-door interview with Kurt Volker, a former special envoy to Ukraine, as House Democrats proceed with Trump's impeachment investigation on Thursday in Washington. (Jose Luis Magana/The Associated Press)

The hours-long briefing took place behind closed doors, the first of a series of events in the probe, none of them public, scheduled within the next 10 days. House investigators want to understand if officials within or outside the State Department played any role or have more information about Trump's efforts to press Zelensky for damaging information about the former U.S. vice-president.

Democratic lawmakers have said they are prepared to subpoena White House records about Trump's July 25 telephone call with Zelensky.

I would say President Zelensky, if it was me, I would recommend that they start an investigation into the Bidens. - Donald Trump

Charges that Trump pressured Zelensky to investigate the Bidens helped prompt House of Representatives Speaker Nancy Pelosi to announce a formal impeachment investigation last week.

Trump on Tuesday again accused the whistleblower of fabricating the contents of the phone call but then appeared to confirm its most damaging allegation.

"I would say President Zelensky, if it was me, I would recommend that they start an investigation into the Bidens," Trump said.

China should also look into business dealings involving Hunter Biden, he said.

Trump just doesn't learn his lesson. It's illegal to pressure a foreign government to investigate a political rival. <a href="https://t.co/pFh6GpnvHT">https://t.co/pFh6GpnvHT</a> —@RepTimRyan

Congressman Tim Ryan of Ohio was among the Democrats to quickly react to Trump's plea, calling it "illegal."

Michigan congressman Justin Amash, a former Republican now Independent who has been critical of Trump, tweeted: "He's openly challenging our system of checks and balances. In plain sight, he's using the powers of his public office for personal gain and counting on Republicans in Congress to look the other way."

Hunter Biden had business interests in Ukraine and China while his father served as vice-president.

Trump supporters have accused Joe Biden of leading a U.S. effort to remove a Ukraine prosecutor, Viktor Shokin, in a bid to end investigations at Burisma, an energy company in which his son held a board seat.

But Ukraine officials have gone on record stating that any inquiries into potential corruption at Burisma were focused on the 2011-2012 period, two years before the younger Biden joined the board. Shokin was removed by then-president Petro Poroshenko in early 2016 after just over a year in the role.

Without referring to him by name, Trump referred to Shokin on Thursday as a "very tough prosecutor," but British authorities, the European Union and the International Monetary Fund officials accused Shokin at the time of not doing enough investigative work in a country plagued by corruption — the non-governmental organization Transparency International has perennially ranked Ukraine in the bottom 60 of its rankings of countries with respect to corruption.

Trump froze $400 million in U.S. aid to Ukraine shortly before he asked its president for the favour, prompting accusations from Democrats that he had misused U.S. foreign policy for personal gain.

Volker told the House investigators on Thursday that it was unusual for the U.S. to withhold aid to Ukraine, but said he was given no explanation for it.

Inspector general on tap to appear

In addition to Volker, Intelligence Community Insp. Gen. Michael Atkinson will testify at an intelligence committee hearing on Friday. Marie (Masha) Yovanovitch, the U.S. ambassador in Kyiv until Trump recalled her in May before her normal term was up, will meet with committee staff on Oct. 11.

When asked about Yovanovitch on Thursday, Trump said, "I heard very bad things about her for a long period of time," without elaborating.

National Intelligence Insp. Gen. Michael Atkinson leaves a closed intelligence briefing on Sept. 26 on Capitol Hill in Washington. He is scheduled to return to Congress on Friday. (Jacquelyn Martin/The Associated Press)

Some of Trump's Republican allies have joined Trump in seeking to discredit California Democrat Adam Schiff after a New York Times report on Wednesday said that the whistleblower had contacted the committee for advice on how to proceed.

Both Republicans and Democrats familiar with the process said, however, that Schiff's panel had acted properly in advising the person to contact an inspector general and seek legal counsel.

The Democratic probe could lead to approval of articles of impeachment — or formal charges — against Trump in the House. A trial on whether to remove Trump from office would then be held in the Republican-controlled Senate. Republicans, however, have shown little appetite for ousting him.

House Republican leader Kevin McCarthy wrote to Democratic House Speaker Pelosi on Thursday 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 authorizing 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, McCarthy said.