A top US diplomat has said that President Trump directly asked about a Ukrainian investigation into his Democratic rival Joe Biden.

Speaking at the first public hearing in the impeachment inquiry against President Trump, the acting US ambassador to Ukraine, Bill Taylor said a member of his staff had overheard a telephone call in which the president inquired about the investigations into Joe Biden.

The US President is accused of withholding US military aid to Ukraine in order to pressurize the country's new president to publicly announce a corruption inquiry into Joe Biden.

He also denied any wrongdoing and has called the inquiry a witch-hunt.

NNR/Hassan