President Donald Trump has dismissed a whistleblower allegation that he made a promise to a foreign leader - believed to be Ukraine's Volodymyr Zelensky - calling it a "ridiculous story".
He said his talks with leaders were always "totally appropriate".
Reports say Mr Trump wanted Ukraine to investigate Joe Biden and his son - who was on a Ukrainian gas company board - in return for more US military support.
Mr Biden is frontrunner to be the Democrat's 2020 presidential candidate.
"If these reports are true, then there is truly no bottom to President Trump's willingness to abuse his power and abase our country," Mr Biden wrote in a statement.
He called on the president to "immediately release" a transcript of the phone call "so that the American people can judge for themselves".
In its report on the complaint by the whistleblower, the Washington Post said the intelligence official had found Mr Trump's comment to the foreign leader "so troubling" that they went to the department's inspector general.
The Wall Street Journal, meanwhile, quoted sources as saying Mr Trump had urged Mr Zelensky about eight times to work with his lawyer Rudy Giuliani on an investigation into Mr Biden's son, but had not offered anything in return.
On Friday, House Speaker Nancy Pelosi said that reports of the complaint raised "grave, urgent concerns" for US national security.
Mr Trump and Mr Zelensky spoke by phone on 25 July. The whistleblower's complaint is dated 12 August.
What did Trump say?
He described the complaint as "just another political hack job".
"It's a ridiculous story. It's a partisan whistleblower. He shouldn't even have information. I've had conversations with many leaders. They're always appropriate," he said, speaking alongside Australia's leader Scott Morrison in the White House.
He also called for Mr Biden's finances to be scrutinised.
"It doesn't matter what I discussed, someone ought to look into Joe Biden's billions of dollars and you wouldn't look into that because he's a Democrat," he told reporters.
On Thursday, Mr Trump wrote on Twitter that he knew all his phone calls to foreign leaders were listened to by US agencies.
Ukraine says Mr Trump and Mr Zelensky will meet next week in New York during the UN General Assembly.
What do we know about the complaint?
Democrats are trying to get the complaint turned over to Congress, with many details still unknown.
Intelligence Community Inspector General Michael Atkinson said the complaint consists of a "serious or flagrant problem, abuse or violation of the law" that involves classified information, a letter to lawmakers revealed.
The acting director of national intelligence, Joseph Maguire, has so far refused to share any details of the complaint with lawmakers, leading to an outcry among Democrats.
Under US law, if the complaint is considered to be of "urgent concern", and if the inspector general considers the complaint to be "credible", then the department head is expected to share the information with Congress within seven days.
What other Ukraine allegations are there?
Earlier this month, before the whistleblower's complaint came to light, House Democrats launched an investigation into Mr Trump and Mr Giuliani's interactions with Ukraine.
Three Democratic panel heads - Eliot Engel (foreign affairs), Adam Schiff (intelligence) and Elijah Cummings (oversight) - said Mr Trump and Mr Giuliani had attempted "to manipulate the Ukrainian justice system to benefit the president's re-election campaign and target a possible political opponent".
They allege that Mr Trump and Mr Giuliani tried to pressure the Ukrainian government into investigating Joe and Hunter Biden.
Is Ukraine's leader being punished?
Analysis by Jonah Fisher, BBC Kiev correspondent
No-one in Ukraine's presidential administration wants to talk about the 25 July phone call between Mr Trump and Mr Zelensky, or respond to the claims that Mr Trump asked Ukraine to investigate his possible opponent in 2020 in return for continued US military support. There are a few circumstantial clues.
Firstly, the Ukrainian readout of the call refers to Mr Trump being "convinced" that the new government will "complete [the] investigation of corruption cases, which inhibited the interaction between Ukraine and the USA".
Is that a diplomatic reference to the US president asking for a probe into Mr Biden?
There's then the question as to why President Trump has kept Ukraine's new leader at arms' length since his election in April.
Some $250m in US military assistance for Ukraine was mysteriously held up by the White House this summer, and a long-mooted visit for President Zelensky to Washington has still not materialised. Is Ukraine's leader being punished for refusing to play ball?
What has Giuliani said?
Mr Giuliani gave an interview to CNN where he both confirmed, and denied that he had asked Ukraine to investigate Joe Biden.
He explained the contradiction by saying that he had asked Ukraine "to look into the allegations that related to my client, which tangentially involved Joe Biden in a massive bribery scheme".
He also wrote on Twitter that "a President telling a Pres-elect of a well known corrupt country he better investigate corruption that affects US is doing his job".
Mr Trump and his fellow Republicans have questioned whether it represented a conflict of interest that Hunter Biden had served on the board of Burisma, a Ukrainian gas company.
In May, Ukraine's prosecutor general said there was no evidence of wrongdoing by Joe or Hunter Biden.
WATCH: Rudy Giuliani denies that he asked Ukraine to investigate Joe Biden, before admitting to Chris Cuomo seconds later: "Of course I did!" pic.twitter.com/flcierpKGu— Axios (@axios) September 20, 2019