Asked how long travel restrictions that took effect Monday on South Africa and seven other southern African countries would remain in place, Biden said it "kind of depends."

The US president, who was boarding Air Force One at Minneapolis-Saint Paul International Airport, said he would see "week to week to determine what we need, the state of affairs."

"We're going to learn a lot more in the next couple weeks about the lethality of this virus, about how much it spreads, what we have to control it, etcetera," he told reporters.

Asked if any expansion of the travel restrictions to other countries could be made suddenly, as happened under former Republican president Donald Trump, Biden said: "Unlike Trump I don't shock our allies."