US 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, reports BBC.
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.