Geneva: The new COVID-19 variant, first found in South Africa this week, has been renamed as Omicron and designated as a "Variant of Concern" by the World Health Organisation.
Omicron now joins the globally-dominant delta variant, its weaker rivals Alpha, Beta and Gamma in the most-troubling category of Covid-19 variants.
The WHO has said that the Omicron may spread more quickly than other forms, and preliminary evidence suggested there is an increased risk of reinfection.
Epidemiologists have warned travel curbs may be too late to stop Omicron from circulating globally.
"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.
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 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.
"This variant has a large number of mutations, some of which are concerning," the WHO said, pointing to worrying characteristics.
"Preliminary evidence suggests an increased risk of reinfection with this variant, as compared to other VOCs."
It said the number of Omicron cases appeared to be increasing in almost all provinces of South Africa.
The UN health agency decided to name the variants after the letters of the Greek alphabet, to avoid the countries that first detected them being stigmatised.
The WHO on Friday called on countries to increase their surveillance and virus sequencing efforts to better understand circulating variants.
Apart from South Africa, Omicron has been detected in Israel in a person coming from Malawi; Botswana; Belgium and Hong Kong.
Besides Delta, Omicron and the three other VOCs, there are currently two lower variants of interest and below that, a further seven under monitoring.