The UN agency, in technical advice to its 194 member states, urged them to accelerate vaccination of high-priority groups and to "ensure mitigation plans are in place" to maintain essential health services.

"Omicron has an unprecedented number of spike mutations, some of which are concerning for their potential impact on the trajectory of the pandemic," the WHO said. "The overall global risk related to the new variant of concern Omicron is assessed as very high."

Further research is needed to better understand Omicron's potential to escape protection against immunity induced by vaccines and previous infections, it said, adding that more data was expected in coming weeks.

"Covid -19 cases and infections are expected in vaccinated persons, albeit in a small and predictable proportion", it added.