Donald Trump declined to say whether he pressed Ukraine's leader to investigate political rival Joe Biden's son on Friday, as he sought to downplay a potentially explosive whistleblower complaint.

The US president was responding to reports that the complaint, details of which are not yet public, centred on a “promise” he made to another foreign leader and the country of Ukraine.

Mr Trump’s allies have been calling for Ukraine to investigate Mr Biden, the Democrat front-runner for the 2020 election, and his son Hunter Biden, who worked for a Ukrainian gas company.

The complaint came from a US intelligence figure and was judged “urgent” enough to be handed to Congress by an internal watchdog, but America’s top intelligence official has refused to do so.

It has raised the question of whether the US president asked another foreign leader to carry out investigations that help him politically - something that would be seen as an abusive of power by critics.