People wait to get their flights at OR Tambo's airport in Johannesburg, South Africa, on Friday. AP

Nearly two years into the COVID-19 pandemic, the world is racing to contain a new coronavirus variant potentially more dangerous than the one that has fueled relentless waves of infection on nearly every continent.

A World Health Organisation panel named the variant "omicron” and classified it as a highly transmissible virus of concern, the same category that includes the predominant delta variant, which is still a scourge driving higher cases of sickness and death in Europe and parts of the United States.

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"It seems to spread rapidly,” US President Joe Biden said on Friday of the new variant, only a day after celebrating the resumption of Thanksgiving gatherings for millions of American families and the sense that normal life was coming back at least for the vaccinated. In announcing new travel restrictions, he told reporters, "I’ve decided that we’re going to be cautious.”

People wait for Covid test at the airport in Johannesburg, South Africa, on Friday. AP

The United States, Australia, Brazil, Canada, Saudi Arabia and Oman became the latest countries on Friday to restrict travel from southern Africa, where a new Covid strain labelled a "variant of concern" has been discovered in a potentially heavy blow to the world's efforts to end the coronavirus pandemic.

The World Health Organisation said the new variant is more infectious than the dominant, highly transmissible Delta strain, and renamed this B.1.1.529 strain of Covid-19 as Omicron.

It has reached Europe with one confirmed case in Belgium after being found in South Africa, Botswana and then in Hong Kong.

Anxious tourists in Johannesburg rushed to the airport to catch a last flight out as nations across the globe started shutting their doors, while many worried the new variant could be more resistant to vaccines.

People wait at OR Tambo's airport in Johannesburg, South Africa', on Friday. AP

Omicron's actual risks are not understood. But early evidence suggests it carries an increased risk of reinfection compared with other highly transmissible variants, the WHO said. That means people who contracted COVID-19 and recovered could be subject to catching it again. It could take weeks to know if current vaccines are less effective against it.

In response to the variant's discovery in southern Africa, the United States, Canada, Russia and a host of other countries joined the European Union in restricting travel for visitors from that region, where the variant brought on a fresh surge of infections.

The White House said the US will restrict travel from South Africa and seven other countries in the region beginning Monday. Biden issued a declaration later Friday making the travel prohibition official, with exceptions for US citizens and permanent residents and for several other categories, including spouses and other close family.