Donald Trump has heaped further pressure on Boris Johnson to cooperate with an investigation into the origins of the Russia collusion inquiry.

It came as the US president publicly called for Ukraine and China to launch corruption investigations into Joe Biden, his leading Democrat opponent in 2020.

Both moves formed part of a concerted fightback by Mr Trump after Democrats in Congress announced an impeachment inquiry against him.

Mr Trump is believed to have viewed the ascension of new conservative leaders in both the UK and Australia as an opportunity to pursue his quest to uncover the origins of Robert Mueller's investigation into Russian election interference.

He has described the process as "investigating the investigators."

The US president spoke with Mr Johnson, and Australian prime minister Scott Morrison, in phone calls early in their premierships.