US President Joe Biden moved to reassure the nation on Monday that the United States was doing all it could to stay ahead of the Omicron coronavirus variant and encouraged Americans to get a booster shot or fully vaccinated.

Omicron is a "cause for concern, not a cause for panic", Biden said in an address from the White House with Dr Anthony Fauci standing behind him.

The variant, first identified on Nov 25 in South Africa, hasn't yet been detected in the US. But Biden said that "sooner or later we are going to see cases of this new variant in the United States".

Cases of the Omicron variant have been found in Botswana, Britain, Italy, Canada, Germany, Belgium, Israel, the Netherlands, Australia, the Czech Republic and Hong Kong.

The World Health Organization (WHO) on Friday called Omicron a variant of "concern''. It is the first time a strain of coronavirus has been classified so severely since the Delta variant.

During a meeting on Sunday with the White House COVID response team, Fauci, director of the National Institute of Allergy and Infectious Diseases and Biden's chief medical adviser, told the president that it will take about two weeks "to have more definitive information on transmissibility, severity and other characteristics of the variant".

On Monday, Fauci told Good Morning America that any speculation about Omicron is "premature".

The Biden administration on Monday enacted travel restrictions into the US from South Africa and seven neighboring African countries starting Monday. At least 44 countries, including the UK, Australia and Canada, also have placed travel restrictions on southern African countries.

Biden said that the US travel ban was put in place to give the US more time to get people vaccinated.

The decision has been met with criticism from the WHO and South Africa's president, Cyril Ramaphosa, who called them "scientifically unjustified", and some of the country's health officials, who say that they have been unfairly penalized for revealing their data.

In an address to his country Sunday, Ramaphosa said, "The only thing the prohibition on travel will do is to further damage the economies of the affected countries and undermine their ability to respond to, and recover from, the pandemic."

At least 80 million Americans are unvaccinated. Biden has repeatedly called on them to get vaccinated and urged anyone age 18 or older to get a booster shot six months after their last dose of the vaccine.

Earlier this month, the CDC had recommended that people ages 50 and up, as well as those in long-term care facilities, should get a booster. On Monday, it recommended everyone age 18 and older get an additional shot after completing a COVID-19 vaccination.

Biden also urged Americans to return to wearing protective face masks indoors and in public settings as it has waned. But he stopped short calling for states to direct residents to do so.

However, in New York City, the health commissioner and the mayor on Monday urged residents to go back to indoor mask-wearing to stay one step ahead of Omicron.

On Thursday, the Biden administration will unveil its plan to tackle COVID-19 this winter. The president made it clear that there would not be any lockdowns or shutdowns.

"If people are vaccinated and wear their mask, there's no need for lockdown" he said Monday.

It isn't known if Omicron will evade existing COVID-19 vaccines. The president said that in the event that updated vaccines or boosters are needed to fight the variant, they will "accelerate their development and deployment".

Biden said that Fauci believes that the vaccines will provide protection against severe disease, and the new variant and boosters strengthen that protection "significantly".

Drugmakers Pfizer, Moderna and Johnson & Johnson are working with the White House in case contingency plans are needed to develop new vaccines or booster shots to beat Omicron.

The vaccine makers also are testing the vaccines' effectiveness against the new strain. They are designing new boosters that can tackle new strains and one specifically to battle Omicron. Moderna told National Public Radio that it had been preparing for mutations in variants all year.