President Donald Trump has branded a whistleblower allegation that he made a promise to a foreign leader – believed to be Ukrainian President Volodymyr Zelensky – as a “ridiculous story”.

Donald Trump said his talks with leaders were always “totally appropriate”.

According to reports, President Trump wanted Ukraine to investigate Joe Biden and his son, Hunter – who was on a Ukrainian gas company board – in return for more US military support.

Joe Biden is frontrunner to be the Democrat’s 2020 presidential candidate.

He wrote in a statement: “If these reports are true, then there is truly no bottom to President Trump’s willingness to abuse his power and abase our country.”

Joe Biden called on President Trump 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 President 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 President Trump had urged President Zelensky about eight times to work with his lawyer Rudy Giuliani on an investigation into Joe Biden’s son, but had not offered anything in return.

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On September 20, House Speaker Nancy Pelosi said that reports of the complaint raised “grave, urgent concerns” for US national security.

Presidents Trump and Zelensky spoke by phone on July 25. The whistleblower’s complaint is dated August 21.

Donald Trump described the complaint as “just another political hack job”.

Speaking alongside Australia’s leader Scott Morrison in the White House, the president said: “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.”

President Trump also called for Joe Biden’s finances to be scrutinized.

He told reporters: “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.”

On September 19, President Trump wrote on Twitter that he knew all his phone calls to foreign leaders were listened to by US agencies.

Ukraine says President Trump and President Zelensky will meet next week in New York during the UN General Assembly.

Democrats are trying to get the complaint turned over to Congress, with many details still unknown.

Earlier this month, before the whistleblower’s complaint came to light, House Democrats launched an investigation into President Trump and Rudy Giuliani’s interactions with Ukraine.

Three Democratic panel heads – Eliot Engel (foreign affairs), Adam Schiff (intelligence) and Elijah Cummings (oversight) – said Donald Trump and Rudy 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 President Trump and Rudy Giuliani tried to pressure the Ukrainian government into investigating Joe and Hunter Biden.