As the 2020 election approaches, worries are mounting over attempts from abroad to meddle in US politics

President Donald Trump on Monday dismissed the threat of impeachment over allegations that he tried to pressure Ukraine's leader into investigating his 2020 election rival Joe Biden.

Asked if he was taking the threat from some Democrats seriously, Trump answered: "Not at all seriously."

US media reports revealed that an intelligence community whistleblower had filed a report after becoming alarmed at Trump's alleged attempt to pressure Ukrainian President Volodymyr Zelensky in a phone call.

Trump has confirmed that he discussed what he says was corruption by Biden's family during the call but denies applying pressure on Zelensky.

"We had a perfect phone call," he told reporters on arrival at the United Nations in New York. "Everybody knows it's just a Democrat witch hunt."

Trump has spent much of his first term as president fighting off allegations that he benefited from Russian-sponsored interference during his surprise 2016 win over Democratic opponent Hillary Clinton.

As the 2020 election approaches, worries are mounting over further attempts from abroad to meddle in the tense US political landscape.

Against that background, the whistleblower's reported raising of the alarm has thrown Washington into renewed turmoil.

According to some reports Trump tried to pressure Zelensky to go after Biden by temporarily withholding US military aid to Ukraine.

Growing numbers of Democrats believe they should open impeachment proceedings against Trump. However, party leaders have resisted, knowing that the Republicans, who are in lockstep with Trump, would almost certainly prevent an impeachment trial taking place in the Senate.

The White House is under pressure to release a transcript of the call between Trump and Zelensky. Democrats, who control the lower house of Congress, are also seeking to question the whistleblower, whose identity remains secret.