US Democrat presidential hopeful Joe Biden has accused President Donald Trump of an overwhelming abuse of power after reports claimed the president tried to pressure Ukraine into launching an investigation into the former vice-president and his son.

Over the weekend, a whistleblower in the US intelligence service alleged that last July, Mr Trump made what he said was an inappropriate phone call to the Ukrainian president, Volodymyr Zelensky.

The Wall Street Journal and other news outlets reported that Mr Trump repeatedly asked President Zelensky to investigate unsubstantiated charges that Mr Biden, while US vice president, threatened to withhold US aid unless a prosecutor who was looking into a gas company in which Mr Biden's son was involved, was fired.

The US Intelligence services has refused to release details of the whistleblower's complaint to Congress.

Mr Biden has admitted threatening to withhold aid unless the prosecutor was dismissed.

But this demand was also made by the wider US government, the European Union and other international institutions for the prosecutor's alleged failure to pursue major corruption cases.

While campaigning in Iowa, a visibly angry Mr Biden said: "This appears to be an overwhelming abuse of power. To get on the phone with a foreign leader who is looking for help from the United States and ask about me and imply things … this is outrageous."

"Trump is using this because he knows I'll beat him like a drum and is using the abuse of power and every element of the presidency to try to do something to smear me," said the former vice-president who served under Barack Obama.

President Trump denied doing anything improper.

He wrote in a series of tweets  that his conversation with Mr Zelensky was "perfectly fine and routine."