Image copyright Getty ImagesImage caption Vice-President Biden in 2016 with his son Hunter looking on
President Donald Trump has repeatedly accused his Democratic White House challenger Joe Biden of wrongdoing in regards to Ukraine and China while he was vice-president, which he denies.
The issue has resurfaced following a New York Post article about an alleged email in which an adviser from a Ukrainian energy company apparently thanked Mr Biden's son, Hunter, for inviting him to meet his father.
Asked about the allegations, Joe Biden told a reporter it was a "smear campaign".
Claims of influence-peddling are common in Washington DC and Mr Trump's children have also been accused of conflicts of interest in lucrative business deals overseas. They, too, deny wrongdoing.
Mr Biden will face Mr Trump, a Republican, in next month's presidential election.
An article has appeared in the New York Post focused on an email from April 2015, in which an adviser to Burisma apparently thanked Hunter Biden for inviting him to meet his father in Washington.
Hunter, Joe Biden's second son, was a director on the board of Burisma - a Ukrainian-owned private energy company while his father was the Obama administration's pointman on US-Ukrainian relations. Hunter was one of several foreigners on its board.
The New York Post article did not provide evidence that the meeting ever took place. The Biden election campaign said there was no record of any such meeting on the former vice-president's "official schedule" from the time.
But in a statement to Politico , the campaign also acknowledged that Mr Biden could have had an "informal interaction" with the Burisma adviser that did not appear on his official schedule, though it said any such encounter would have been "cursory".
"Investigations by the press, during impeachment, and even by two Republican-led Senate committees whose work was decried as 'not legitimate' and political by a GOP colleague, have all reached the same conclusion: that Joe Biden carried out official US policy toward Ukraine and engaged in no wrongdoing," said Andrew Bates, a spokesman for Mr Biden.Video caption What we know about Biden-Ukraine corruption claims
The campaign also decried the New York Post story as "Russian disinformation", though it did not say the emails were bogus.
The New York Post article has been shared by President Trump and his allies. Two of his former advisers, Steve Bannon and Rudy Giuliani, were involved in providing the story and the hard drive containing the alleged emails, to the newspaper.
Other US media say they have been unable to verify the authenticity of the emails.
Hunter joined Burisma in 2014, and remained on the board until April 2019, when he decided to leave.
The New York Post also cited a purported email from Hunter Biden indicating that he was receiving a $10m annual fee from a Chinese billionaire for "introductions alone", though it is unclear who was involved in the alleged introductions.
Hunter flew in 2013 aboard Air Force Two with his father, who was US vice-president, on an official visit to Beijing, where the younger Biden met investment banker Jonathan Li.
Hunter told the New Yorker he just met Mr Li for "a cup of coffee", but 12 days after the trip a private equity fund, BHR Partners, was approved by the Chinese authorities. Mr Li was chief executive and Hunter was a board member. He would hold a 10% stake.
Image copyright Getty ImagesImage caption Joe Biden walks out of Air Force Two with his granddaughter and son, Hunter Biden
BHR is backed by some of China's largest state banks and by local governments, according to US media.
Hunter Biden's lawyer said he had joined the board in an unpaid position "based on his interest in seeking ways to bring Chinese capital to international markets".
His lawyer also said his client did not acquire his financial stake in BHR until 2017, after his father had left office in the US.
Hunter resigned from the board of BHR in April 2020, but still held his 10% stake in BHR as of July this year, according to the company report.
President Trump and his allies have accused Joe Biden of wrongdoing because he pushed, while vice-president, for the Ukrainian government to fire its top prosecutor, who was investigating the company for which Hunter worked.
In 2016, Joe Biden called for the dismissal of the Ukrainian prosecutor Viktor Shokin, whose office had Burisma and other companies under investigation.
However, other Western leaders and major bodies that give financial support to Ukraine also wanted the prosecutor dismissed because they believed he was not active enough in tackling corruption.
In 2019, details emerged of a phone call President Trump had made to the president of Ukraine, in which he had urged the Ukrainian leader to investigate the Bidens.
This led to charges by the Democrats that Mr Trump was trying to illegally pressure Ukraine to help damage his election rival, resulting in impeachment by the House of Representatives.
Mr Trump denied he'd done anything wrong, and he was later acquitted by the Republican-controlled US Senate.
Image copyright Getty ImagesImage caption President Trump holds a copy of The Washington Post the day after the US Senate acquitted him on two articles of impeachment
No criminal activity has been proven, and no evidence has emerged that Joe Biden did anything to intentionally benefit his son. But it has raised questions about potential conflicts of interest.
A senior State Department official raised such concerns as far back as 2015.
US Republican lawmakers launched an investigation and found that Hunter's work for the Ukrainian firm was "problematic" - but there wasn't evidence that US foreign policy was influenced by it.
No criminal charges were proven against Burisma either. The company issued a statement in 2017 saying "all legal proceedings and pending criminal allegations" against it were closed.
Last year, Yuriy Lutsenko, the prosecutor in Ukraine who succeeded Viktor Shokin, told the BBC that there was no reason to investigate the Bidens under Ukrainian law.
There's nothing illegal about sitting on a board of a company whilst family members serve in government, although Hunter Biden now says that serving on the Burisma board may have been "poor judgement".
Hunter Biden's lawyers said in statement in October 2019 that he had undertaken "these business activities independently. He did not believe it appropriate to discuss them with his father, nor did he."
Hunter told the New Yorker magazine that on the only occasion he had mentioned Burisma: "Dad said, 'I hope you know what you are doing.'"
Amid all the scrutiny, the Democratic White House nominee said last year that if he is elected president, no-one in his family will hold a job or have a business relationship with a foreign corporation or foreign government.