Japan said on Monday it would close its borders to foreigners, as the world's third-largest economy joined Israel in taking the toughest measures against the Omicron variant of coronavirus, which the WHO said carried a "very high" global risk of surges.

Markets regained some composure as investors awaited further details of the variant, after having sunk last week on fears that it could bring fresh curbs, threatening a nascent economic revival from a two-year pandemic.

Potentially more contagious than prior variants, Omicron, first identified in southern Africa, has been found in Australia, Belgium, Botswana, Britain, Canada, Denmark, France, Germany, Hong Kong, Israel, Italy, the Netherlands and Scotland.

The Omicron variant is likely to spread internationally, bringing "severe consequences" in some areas, the World Health Organization (WHO) advised its 194 member nations.

"The overall global risk related to the new variant of concern Omicron is assessed as very high," said the world health body, which has warned that understanding the variant's level of severity could take time running into weeks.

As a precaution to avert a worst-case scenario, Japan will close its borders to foreigners from Tuesday, Prime Minister Fumio Kishida said, adding that he was ready to face criticism for being too cautious.