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.
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.