Donald Trump last night ramped up his accusations that Joe Biden demanded Ukraine fire a prosecutor investigating his son Hunter's business dealings.

The US president lashed out amid an escalating controversy sparked by a whistleblower's claim that Mr Trump himself had tried to pressure the Ukrainian leader to pursue an inquiry into Hunter Biden.

Ukraine is now at the centre of a complex scandal that threatens to damage both the president and his potential Democrat opponent in 2020.

It stems from an internal complaint on Aug 12 by an unknown whistleblower in the US intelligence community.

The complaint centred on a July 25 telephone conversation between Mr Trump and Ukraine's newly elected president Volodymyr Zelensky.

Mr Trump was said to have repeatedly pressured Mr Zelensky to investigate Hunter Biden's dealings in Ukraine, where he was on the board of gas company. According to the Wall Street Journal, Mr Trump did so "about eight times".

Democrats have accused the president, and his lawyer Rudy Giuliani, of trying to coerce Ukraine to pursue Hunter Biden in exchange for military assistance.

Mr Trump, meanwhile, has accused Democrats of trying to distract from allegations he has been making against Joe Biden.

He claims that, in 2016, Mr Biden, then the vice-president, demanded that the Ukrainian government remove the prosecutor looking into the gas company Hunter Biden was associated with.