WASHINGTON – The United States will restrict travel for non-U.S. citizens from South Africa and seven other countries starting Monday, part of a global effort to stem the spread of the heavily mutated omicron variant of Covid-19, according to senior Biden administration officials.

In addition to South Africa, other countries included in the new restrictions are Botswana, Zimbabwe, Namibia, Lesotho, Eswatini, Mozambique and Malawi.

There was no indication Friday of how long the bans will remain in place. President Joe Biden said in a statement that moving forward he will be “guided by what the science and my medical team advises.”

The decision came less than three weeks after the administration lifted pandemic travel restrictions on visitors from more than 30 countries, including South Africa.

Biden was briefed on the variant Friday by White House chief medical advisor Dr. Anthony Fauci, as a growing list of countries issued their own travel bans.

Canada, the European Union and the U.K. all announced restrictions on travelers from southern Africa Friday, even as Belgian officials announced that several cases of omicron variant Covid had already been identified there.

Also on Friday, the World Health Organization assigned the newly identified variant the Greek letter omicron and formally recognized the strain, previously referred to as lineage B.1.1.529, as a “variant of concern.”

Health experts are deeply concerned about the transmissibility of the omicron variant given that it has an unusual constellation of mutations and a profile that is different from other variants of concern. It is not clear how severe infections would be for vaccinated patients.

It is feared a sharp upswing of Covid cases in South Africa’s Gauteng province — where the heavily mutated strain of the virus was first identified — could mean it has greater potential to escape prior immunity than other variants.

In a statement announcing the travel ban, Biden urged already immunized Americans to get their booster shots, and parents to take advantage of the new vaccine doses approved for children aged 5 to 11.

The emergence of this new strain in South Africa, Biden said, also serves to underscore the importance of making vaccines accessible to people all over the world. To that end, he urged members of the World Trade Organization to waive intellectual property protections for Covid vaccines.

The designation of a new variant of concern coupled with mounting alarm from health officials sent global markets into a tailspin on Friday. Oil prices took heavy losses on the news.

Airline and other travel stocks plunged Friday. The fresh travel restrictions followed reports of the new variant in places as distant as Botswana, Belgium, Israel and Hong Kong.

The new restrictions come just as carriers and aerospace manufacturers like Boeing have been upbeat about a resurgence in travel demand, particularly international routes next year.

Flights between the U.S. and South Africa are limited compared with other international destinations, but the sudden changes in travel rules make it difficult for customers to book and could further delay the return of lucrative international business travel.

There are 122 flights between the U.S. and South Africa scheduled for December, according to aviation consulting firm Cirium.

United, which has the most scheduled service with 87 flights, is set to resume nonstop flights between its Newark, New Jersey, hub and Cape Town next month. A spokeswoman said no changes are currently planned.

Delta has 35 scheduled flights between the U.S. and South Africa in December.

— CNBC’s Sam Meredith and Robert Towey contributed reporting.