Doctors treating US President Donald Trump for Covid-19 sent conflicting signals about the severity of his condition on Sunday, hours before the president surprised supporters gathered outside the hospital with an impromptu motorcade.

Trump, 74, wore a mask as he waved from the back seat of a black SUV that crawled in a caravan of vehicles in front of the Walter Reed National Military Medical Center outside Washington, while supporters waving Trump 2020 flags chanted: "USA! USA!"