US President Donald Trump has publicly called on Ukraine and China to investigate Joe Biden and his son.
Ex-Vice-President Joe Biden is a Democratic frontrunner in the 2020 presidential race, and a central figure in the push to impeach Mr Trump.
"China should start an investigation into the Bidens," said Mr Trump, in reference to Mr Biden's son, Hunter.
The Democrats accuse the US president 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 says he has done nothing wrong and the inquiry is a witch hunt.
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, but offered no 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, and the elder Biden was making frequent trips to the 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, several Western governments 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 the July 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.
"So I would say that President Zelensky, if it were me, I would recommend that they start an investigation into the Bidens because nobody has any doubt that they weren't crooked. That was a crooked deal one hundred percent."
How has Biden 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.
The remark was widely condemned as a request for a foreign government to assist his campaign. US election laws ban non-US citizens from contributing to presidential campaigns.
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 with any Chinese officials about the business. However, he reportedly helped arrange for Mr Li to shake hands with his father during his trip to Beijing, which stoked claims of influence-peddling.
This August, Republican Senator and Finance Chairman Chuck Grassley had questioned Hunter's actions on the trip, saying the younger Biden had a "history of investing in and collaborating with Chinese companies, including at least one posing significant national security concerns".
How is the impeachment inquiry progressing?
Ex-US special envoy to Ukraine Kurt Volker has become the first witness to testify to Congress in the impeachment probe of US President Donald Trump.
Mr Volker was among those mentioned in an anonymous whistleblower's complaint about Mr Trump's call to the president of Ukraine.
The Republican foreign policy expert addresses three Democrat-led committees behind closed doors on Thursday.
Mr Volker, the former US envoy to Nato, was testifying to the House of Representatives Intelligence, Oversight and Reform and Foreign Affairs committees.
He was expected to be questioned about his role in Mr Trump's effort to have Ukraine investigate the Bidens.
The whistleblower's complaint said that one day after the Trump-Zelensky call Mr Volker and US ambassador to the EU, Gordon Sondland, met the Ukrainian president to advise him on how to "navigate" Mr Trump's request.
Mr Volker resigned from the Department of State last week after being named in the whistleblower complaint.
Secretary of State Mike Pompeo had sought to block testimony from Mr Volker and four other Department of State officials whom Democrats wish to interview.
In advance of his testimony, Mr Volker handed over documents to the committee, including text messages with Mr Trump's personal lawyer Rudy Giuliani.
Mr Giuliani - who is not a US government official - took to Twitter on Thursday morning to release screen shots of several text messages sent from Mr Volker to him.
"Kurt did nothing wrong," he said in a tweet.
Crooked Dems not letting Republicans subpoena witnesses and maybe even question witness. This is a Star Chamber, illicit and part of their conspiracy to violate constitutional rights condoned by their media lamb dogs. Kurt did nothing wrong. pic.twitter.com/iaplXBnJ1x— Rudy Giuliani (@RudyGiuliani) October 3, 2019
Quick facts on impeachment
- Impeachment is the first part - the charges - of a two-stage political process by which Congress can remove a president from office
- If the House of Representatives votes to pass articles of impeachment, the Senate is forced to hold a trial
- A Senate vote requires a two-thirds majority to convict - unlikely in this case, given that Mr Trump's party controls the chamber
- Only two US presidents in history - Bill Clinton and Andrew Johnson - have been impeached but neither was convicted and removed
- President Nixon resigned before he could have been impeached