The UN health agency, in a technical paper issued to member states, said "considerable uncertainties remain about the variant that was first detected days ago in southern Africa.
But it said it is possible the variant has mutations that could enable it to escape an immune-system response and boost its ability to spread from one person to another.
Depending on these characteristics, there could be future surges of COVID-19, which could have severe consequences, depending on a number of factors, including where surges may take place, it added.
The overall global risk ... is assessed as very high.
The warning came as a widening circle of countries around the world reported cases of the variant and moved to slam their doors shut while scientists race to figure out just how dangerous this version might be.
Japan announced it would bar entry of all foreign visitors, joining Israel in doing so just days after the variant was identified by researchers in South Africa. Morocco banned all incoming flights.
Other countries, including the U.S. and European Union members, have moved to prohibit travelers arriving from southern Africa.
If the virus proves dangerous as feared, "the impact on vulnerable populations would be substantial, particularly in countries with low vaccination coverage, WHO said, noting that no deaths linked to omicron have been reported so far.
Spain became one of the latest countries to report its first confirmed case of the variant. While the vast majority of infections have been seen in travelers arriving from abroad, cases in Portugal and Scotland have raised fears that the variant may already be spreading locally.
Many of us might think we are done with COVID-19. It's not done with us, warned Tedros Adhanom Ghebreyesus, WHO's director-general.
Days after the variant sent a shudder through the financial world nearly two years into the pandemic that has killed over 5 million people, markets had mixed reactions Monday, with European stocks and oil prices rebounding and Wall Street opening higher, while Asian markets fell further.
U.S. President Joe Biden called the omicron variant a cause for concern but not a cause for panic. He said he is not considering any widespread U.S. lockdown and instead urged vaccinations and mask-wearing.
The infections have underscored the difficulty in keeping the virus in check in a globalized world of jet travel and open borders. Yet, many countries are trying to do just that, against the urging of the WHO, which noted that border closings often have limited effect and can wreak havoc on lives and livelihoods.
Some argued that such restrictions could provide valuable time to analyze the new variant. Little is known about it, including whether it is more contagious, more likely to cause serious illness or more able to evade vaccines.
While the initial global response to COVID-19 was criticized as slow and haphazard, the reaction to the new variant came quickly.
This time the world showed it is learning, said EU Commission President Ursula von der Leyen, singling out South African President Cyril Ramaphosa for praise. South Africa's analytic work and transparency and sharing its results was indispensable in allowing a swift global response. It no doubt saved many lives.
Late last week, von der Leyen successfully pushed the 27-nation EU to agree to ban flights from seven southern African nations, similar to what many other countries are doing.
Cases had already been reported in EU nations Belgium, Denmark and the Netherlands before Portuguese authorities identified 13 omicron infections among members of the Belenenses professional soccer team. Authorities reported one member had recently traveled to South Africa. A game over the weekend had be abandoned at halftime for lack of players.
Spain's first document case was detected in a traveler who returned Sunday from South Africa after making a stopover in Amsterdam, one of Madrid's major public hospitals tweeted.
And after Scotland reported its first six cases, First Minister Nicola Sturgeon warned that there might already be some community transmission of this variant.
Taking no chances, Japan, which has yet to detect any omicron cases, reimposed border controls that it had eased earlier this month.
We are taking the step as an emergency precaution to prevent a worst-case scenario in Japan, Prime Minister Fumio Kishida said. The new measures begin Tuesday.
Israel likewise decided to bar entry to foreigners, and Morocco said it would suspend all incoming flights for two weeks starting Monday.
And Britain expanded its COVID-19 booster program to everyone 18 and older, making millions more people eligible. Up until now, booster shots were available only to those 40 and over and people particularly vulnerable to the virus. The U.K. has reported about a dozen omicron cases.
Despite the global worry, scientists cautioned that it is still unclear whether omicron is more alarming than other versions.
So far, doctors in South Africa are reporting patients are suffering mostly mild symptoms, but they warn that it is still early. Also, most of the new cases are in people in their 20s and 30s, who generally do not get as sick from COVID-19 as older patients.
We've seen a sharp increase in cases for the past 10 days. So far they have mostly been very mild cases, with patients having flu-like symptoms: dry coughs, fever, night sweats, a lot of body pains, said Dr. Unben Pillay, a general practitioner in Gauteng province, where 81% of the new cases have been reported.
The variant has provided further proof of what experts have long been saying: that no continent will be safe until the whole globe has been sufficiently vaccinated. The more the virus is allowed to spread, the more opportunities it has to mutate.
The emergence of the omicron variant has fulfilled, in a precise way, the predictions of the scientists who warned that the elevated transmission of the virus in areas with limited access to vaccine would speed its evolution, said Dr. Richard Hatchett, head of CEPI, one of the founders of the U.N.-backed global vaccine sharing initiative COVAX