Senior US government scientist Anthony Fauci said Friday Washington wanted more data on a worrisome new Covid-19 variant first detected in southern Africa before joining other countries in banning flights from the region.

Fauci told CNN that while "there's always the possibility" of blocking flights, "you want to make sure there's a basis for doing that. And that's what we're doing right now."

He said there was "no indication" yet that the newly identified strain of Covid-19 had reached the United States.

The emergence of the new variant, first recorded in South Africa, has sent nations rushing to evaluate the threat.

Britain, Germany, Italy and the Netherlands are among nations suspending flights from South Africa and surrounding countries, while Japan will require travelers from the region to quarantine for 10 days.

Fears that the variant could seriously set back global efforts to combat the Covid-19 pandemic, battering already vulnerable economies, sent stocks plunging on Wall Street, down at midday by nearly 1,000 points.

As scientists race to evaluate the heavily mutated strain -- designated for now by its scientific name B.1.1.529 -- there are fears it may prove more infectious than the highly contagious Delta, and possibly more resistant to vaccines.

"As soon as we find out more information, we will make a decision as quickly as we possibly can," said Fauci, who is President Joe Biden's top adviser on the coronavirus.

"You don't want to say you're going to do it until you have some scientific reason to do it. That is the reason why we're rushing now to get that scientific data."

Some in South Africa criticized the travel bans as premature, saying the country was being punished for its transparency on the new strain.

The World Health Organization (WHO) has said it could take weeks to understand the variant and cautioned against imposing travel curbs while scientific evidence was still scant.