The new Covid-19 Omicron variant poses a "very high" overall risk of global surge in infections, the World Health Organization (WHO) warned on Monday.

"If another major surge of Covid-19 takes place driven by Omicron, consequences may be severe," WHO said in a technical note.

"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.

To date, no deaths linked to Omicron has been reported, though further research was needed to assess Omicron's potential to escape protection against immunity induced by vaccines, it 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 said.

Also read:  What you need to know about the new deadlier Omicron variant of COVID-19

The variant was first reported to WHO on Nov. 24 from South Africa, where coronavirus infections have suddenly shot up.

The variant has now spread around the world, with new cases found in UK, Germany, Italy, 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.

Overall, there are considerable uncertainties in the magnitude of immune escape potential of Omicron. More data was expected in coming weeks, the report said.

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