The WHO says the Omicron variant has now been identified in around two dozen countries.
The head of the WHO spoke on Wednesday about the spread.
"The emergence of the omicron variant has understandably captured global attention. At least 23 countries from five of six WHO regions have now reported cases of omicron, and we expect that number to grow. WHO takes this development extremely seriously and so should every country, but it should not surprise us."
The WHO chief also said more information is being collected about the new strain.
Despite mounting uncertainties around Omicron, Ghebreyesus called for people to get vaccinated as the shots help save lives.
Japan announced Wednesday that it had confirmed its second case of the Omicron variant, prompting Tokyo to implement tougher travel restrictions.
The Japanese government will deny entry to all foreigners from ten African countries feared to have outbreaks of the variant, including South Africa.
It also asked airlines to stop taking reservations for inbound international flights, meaning all Japanese citizens and foreign residents will not be allowed to enter the country.
Japan says the measures will remain in place for the time being.
In Europe, at least 11 countries have confirmed Omicron cases, with the European Commission President saying the EU needs to "hope for the best, and prepare for the worst.
"Therefore, until we know what's going on - and this will take two to three weeks, that is what the scientists tell us - we have to do everything possible to make the best out of the time we have until we have certainty about the characteristics of transmissibility and severity of omicron."
As of Wednesday, 59 cases of the new variant had been identified among EU member-states, with most of them having recently been in southern Africa.
The U.S. has also reported its first case of the new strain.
The infected person is a fully vaccinated traveler who returned to California from South Africa on November 22nd.
The U.S. Centers for Disease Control and Prevention added that no other potential Omicron cases are being investigated, and all close contacts of the infected person had tested negative.
Kim Hyo-sun, Arirang News.