The variant was first detected in southern Africa, which has led to a surge in infections in South Africa and other countries in the southern part of the continent.
"Based on the evidence presented indicative of a detrimental change in Covid-19 epidemiology... the WHO has designated B.1.1.529 as a variant of concern (VOC), named Omicron," the UN health agency said in a statement.
This puts Omicron into the most-troubling category of COVID-19 variants besides the globally-dominant Delta.
Belgium on Friday detected its first case of the new variant of COVID-19, which also marks the first infection in Europe.
"We have a case that is now confirmed of this variant," Belgian Health Minister Frank Vandenbroucke had said during a media conference.
Meanwhile, the French government announced on Friday to immediately suspend flights coming from the southern parts of Africa following the detection of a new Covid-19 variant in South Africa.
In a statement published Friday by the prime minister's office, travellers from South Africa, Lesotho, Botswana, Zimbabwe, Mozambique, Namibia and Eswatini, will not be able to enter French territory, regardless of their vaccination status.
The WHO said it could take several weeks to complete studies of Omicron to understand if there are any changes in transmissibility, severity or implications for Covid vaccines, tests and treatments.
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