US President Donald Trump, already facing an impeachment inquiry after urging Ukraine to investigate his political rival Joe Biden, has now urged China to do likewise.
He said both China and Ukraine should look into Mr Biden, a Democratic presidential frontrunner, and his son.
"China should start an investigation into the Bidens," said Mr Trump.
The Democrats accuse Mr Trump of dangling military aid as a way to press Ukraine to dig dirt on Mr Biden.
Mr Biden responded by tweeting: "The idea of Donald Trump attacking anyone's integrity is a joke."
A 25 July phone call between Mr Trump and Ukrainian President Volodymyr Zelensky triggered an intelligence community whistleblower complaint and the impeachment inquiry.
But Mr Trump said on Thursday that the inquiry, which could culminate in a congressional attempt to remove him from office, is "crap".
In a late-night tweet, Mr Trump added that he had a duty to investigate alleged corruption "and that would include asking, or suggesting, other countries to help us out!"
What has Trump accused the Bidens of doing?
Mr Trump has accused Mr Biden and his son Hunter of corruption in their political and business dealings in Ukraine and China, without offering specific evidence.
When Hunter Biden joined Ukrainian natural gas company Burisma in 2014, questions were raised about a potential conflict of interest for his father.
Ukraine was undergoing a political transition after its pro-Russia president was forced out of office, while the elder Biden was the Obama administration's point man for the Eastern European country.
In 2016, Joe Biden pushed the Ukrainian government to fire its top prosecutor, Viktor Shokin, whose office had been scrutinising the oligarch owner of Burisma.
In a speech last year at a think tank, Mr Biden boasted of having forced Mr Shokin out by threatening to withhold a billion-dollar loan guarantee to Ukraine.
Mr Trump and his allies accuse Mr Biden of acting to protect his son. However, other Western officials and major financers of Ukraine's government also wanted Mr Shokin dismissed because he was seen as a barrier to anti-corruption efforts.
Last week, the Ukrainian prosecutor general who took over from Mr Shokin told the BBC there was no evidence of wrongdoing by Joe or Hunter Biden.
What did Trump say on Thursday?
When asked what Mr Trump sought as a "favour" from Ukrainian President Volodymyr Zelensky in this summer's phone call, Mr Trump responded: "Well, I would think that if they were honest about it, they'd start a major investigation into the Bidens. It's a very simple answer.
"They should investigate the Bidens," he said, speaking to reporters on the lawn of the White House.
"Likewise, China should start an investigation into the Bidens because what happened to China is just about as bad as what happened with Ukraine," he continued.
Mr Trump also suggested again without offering evidence that Mr Biden had "scammed" other countries and was responsible for China's "sweetheart" trade relationship with the US.
The head of the Federal Election Commission responded by tweeting a reminder that it is illegal for anyone to solicit anything of value from a foreign national in connection with a US election.
How have the Bidens responded?
A spokeswoman for Mr Biden's 2020 campaign accused Mr Trump of "flailing and melting down on national television, desperately clutching for conspiracy theories that have been debunked and dismissed by independent, credible news organisations".
Deputy Campaign Manager Kate Bedingfield's statement compared Mr Trump's latest comments to his "'Russia, if you're listening' moment from 2016 - a grotesque choice of lies over truth and self over the country".
The quote is a reference to Mr Trump's first election campaign, in which he urged Russia to find more than 30,000 emails deleted from his opponent Hillary Clinton's personal server.
A spokesman for Hunter Biden has told US media that Mr Trump's $1.5bn (£1.2bn) figure is false, and that the younger Biden did not receive any return or compensation from the Chinese firm.
What about the Bidens in China?
In 2013, then vice-president Mr Biden went to China on an official visit, where he met Chinese President Xi Jinping and other officials.
Hunter Biden and his daughter joined the vice-president, who had travelled with family members before.
During the two-day visit, Hunter met a Chinese banker, Jonathan Li, who would eventually become a business partner.
Mr Li founded a private equity fund shortly after the trip, and Hunter was on the board, although a spokesman for the younger Mr Biden told NBC News they did not discuss any business during the trip and the fund had been planned months earlier.
Hunter Biden was also not an equity owner in the fund during his father's term as vice-president, according to the spokesman.
Hunter has denied meeting any Chinese officials about the business. However, he reportedly helped arrange for Mr Li to shake hands with Joe Biden during his trip to Beijing, which stoked claims of influence-peddling.
This August, Republican Senator and Finance Chairman Chuck Grassley questioned Hunter's actions on the trip.
He said the younger Biden had a "history of investing in and collaborating with Chinese companies, including at least one posing significant national security concerns".