No Omicron-linked deaths had yet been reported, though further research was needed to assess its potential to escape protection against immunity induced by vaccines and previous infections, it added.

In anticipation of increased case numbers as the variant, first reported last week, spreads, the UN agency urged its 194 member states to accelerate vaccination of high-priority groups and ensure plans were in place to maintain 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 ...is assessed as very high."

Tedros Adhanom Ghebreyesus, WHO director-general, sounded the alarm at the start of an assembly of health ministers that is expected to launch negotiations on an international agreement on preventing future pandemics.

"The emergence of the highly mutated Omicron variant underlines just how perilous and precarious our situation is," Tedros said. "Omicron demonstrates just why the world needs a new accord on pandemics: our current system disincentivizes countries from alerting others to threats that will inevitably land on their shores."

The new global deal, expected by May 2024, would cover issues such as sharing of data and genome sequences of emerging viruses, and of any potential vaccines derived from research.

'Overwhelming demands'

Omicron was first reported on Nov 24 from South Africa, where infections have risen steeply.

It has since spread to more than a dozen countries, many of which have imposed travel restrictions to try to seal themselves off. Japan on Monday joined Israel in saying it would close its borders to foreigners.

The WHO reiterated that, pending further advice, countries should use a "risk-based approach to adjust international travel measures in a timely manner", while acknowledging that a rise in coronavirus cases might lead to higher morbidity and mortality rates.

"The impact on vulnerable populations would be substantial, particularly in countries with low vaccination coverage," it added.

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

Overall, there were "considerable uncertainties in the magnitude of immune escape potential of Omicron", and more data was expected in coming weeks.