The UN organisation said it could take several weeks to complete studies of the new variant and that the number of Omicron cases appeared to be increasing in almost all provinces of South Africa.
The UN health agency has declared the recently-discovered B.1.1.529 strain of Covid-19 to be a variant of concern, renaming it Omicron.
"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 World Health Organisation stated on Friday.
The WHO said it could take several weeks to complete studies of Omicron to see if there are any changes in transmissibility, severity or implications for Covid vaccines, tests and treatments.
The classification puts Omicron into the most-troubling category of Covid-19 variants, along with the globally-dominant Delta, plus its weaker rivals Alpha, Beta and Gamma.
Nations rushed to ban flights to slow the spread of Omicron, while stock markets and oil prices plunged on fears surrounding the variant, potentially dealing a heavy blow to the global economic recovery.
The change in classification came after a quickly-assembled virtual meeting of the WHO's Technical Advisory Group on SARS-CoV-2 Virus Evolution.
The variant was first reported to the WHO from South Africa on Wednesday.
The first known confirmed Omicron infection was from a specimen collected on November 9. In recent weeks, infections in South Africa have increased steeply, coinciding with the detection.
The WHO said the number of Omicron cases appeared to be increasing in almost all provinces of South Africa.
Flurry of travel bans
Apart from South Africa, Omicron has been detected in Israel in a person coming from Malawi; Botswana; Belgium and Hong Kong.
Despite countries scrambling to ban flights, the WHO earlier cautioned against imposing travel restrictions due to Omicron.
The organisation said countries should take a risk-based and scientific approach when considering travel curbs in light of the variant – but cautioned against restrictions.
Besides Delta, Omicron and the three other VOCs, there are currently two lower variants of interest and below that, a further seven under monitoring.