Markets, however, regained some composure as investors await more details of the variant, after sinking last week on fears 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 South Africa, has been found in Australia, Belgium, Botswana, Britain, Canada, Denmark, France, Germany, Hong Kong, Israel, Italy, and the Netherlands.
It could take "days to several weeks" to understand the level of severity of Omicron, says the World Health Organization (WHO), which flagged it as a "variant of concern".
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 for criticism that he was being too cautious.
"These are temporary, exceptional measures that we are taking for safety's sake until there is clearer information about the Omicron variant," Kishida told reporters.
He did not say how long the restriction would stay. Japanese returning from specified nations would face quarantine in designated facilities, Kishida added.