Former US President Barack Obama on Saturday criticised the response to the Covid-19 pandemic ravaging America, in what was widely regarded as a rare public rebuke of his successor Donald Trump.

"More than anything, this pandemic has fully, finally torn back the curtain on the idea that so many of the folks in charge know what they're doing," Obama told graduates from several dozen historically black colleges and universities at the first ceremony.

That celebration was followed by a second, star-studded prime-time ceremony for high school students in which Obama continued along the same lines, telling students to step up where leaders had failed.

James echoed Obama's call for the students to step up, urging them to "Stay close to home" and help rebuild communities devastated by the virus and shutdowns.

The gravity of Obama's message was underscored by the fact it was the second time in a week he had hit out at the response to the pandemic, having pointedly refused to criticise Trump in public in the three years since leaving office.