To date, no deaths linked to Omicron had been reported, though further research was needed to assess Omicron' s potential to escape protection against immunity induced by vaccines and previous infections, the WHO  said.

" Increasing cases, regardless of a change in severity, may pose overwhelming demands on health care systems and may lead to increased morbidity and mortality. The impact on vulnerable populations would be substantial, particularly in countries with low vaccination coverage," it added.

It has since spread around the world, with new cases found in the Netherlands, Denmark and Australia even as more countries imposed travel restrictions to try to seal themselves off. Japan said on Monday it would close its borders to foreigners, joining Israel in taking the toughest measures.

The WHO, in its latest guidance, reiterated that countries should use a " risk-based approach to adjust international travel measures in a timely manner" . Further advice would be forthcoming, it said.