In a leaked web call Friday night with former members of his administration, Obama also said the Justice Department's decision to drop charges against Michael Flynn, the former Trump national security adviser who pleaded guilty to lying to the FBI in the Russia probe, endangers the rule of law in the US.

In the audio, first obtained by Yahoo News, Obama urges former staffers to join him in rallying behind Joe Biden as he prepares to take on Trump in the November presidential election.

The United States by far leads the world in the number of coronavirus infections, at nearly 1.3 million, and deaths, with more than 77,000.

Trump has been criticized as essentially abdicating any leadership role in guiding the country through one of its worst crises in a century, leaving states on their own to grapple with the pandemic and even bid against each other to obtain critical medical equipment on the open market or abroad.

Critics say Trump, after first downplaying the threat posed by the virus, squandered precious time in February as the pathogen spread in America and his administration did little to stock up on testing kits and other medical gear or to develop a cohesive national strategy.

With an eye to re-election, the president has also been blasted as putting his own political interests before human life by aggressively pushing states to reopen their devastated economies without a clear blueprint for how to do it safely.