United States President Donald Trump is facing allegations he paid little or no income tax for years before he came to power, as his cloudy financial past stoked controversy ahead of the first election debate.

The New York Times alleged the billionaire President paid just US$750 (S$1,030) in federal income taxes in 2016, the year he won the White House, and no income taxes at all in 10 of the previous 15 years because he reported losing more money than he made.

Mr Trump, who dismissed the accusations as "totally fake news", is readying to come face to face with his Democratic opponent Joe Biden in a live debate today.

The New York Times said tax data spanning over 20 years "reveal the hollowness, but also the wizardry, behind the self-made-billionaire image" of the President.