"The overall global risk related to the new variant of concern Omicron is assessed as very high," the World Health Organization advised its 194 member nations, warning of "severe consequences" in some areas.
It said no deaths linked to Omicron variant had been reported so far.
A top South African infectious disease expert said Omicron appears to be more transmissible than previous variants, including to people with immunity from vaccination or prior infection. South African cases are likely to top 10,000 a day this week, rocketing up from 2,858 on Sunday and barely 300 a day two weeks ago, Professor Salim Abdool Karim said.
But he added that it was too early to say whether symptoms were more severe and said existing COVID-19 vaccines are probably effective at stopping Omicron from causing severe illness.
On Sunday, a South African doctor who was one of the first to suspect a new strain said Omicron appeared so far to be producing mild symptoms.
Portugal found 13 cases of the variant at a Lisbon football club. Scotland and Austria also reported their first Omicron cases on Monday.
A number of countries have imposed travel restrictions, including Japan, which described its action as precautionary.
"These are temporary, exceptional measures that we are taking for safety's sake, until there is clearer information about the Omicron variant," Prime Minister Fumio Kishida said.
Health Minister Shigeyuki Goto said tests would determine if a traveller from Namibia was Japan's first Omicron case.
Israel, where a ban on arrivals by foreigners took effect on Monday morning from midnight, has said it will use counter-terrorism phone-tracking technology to combat the new variant.
Australia said it would review plans to re-open borders to skilled migrants and students from Dec. 1, after reporting its first Omicron cases, although Prime Minister Scott Morrison said it was a "bit too early" to reinstate two-week hotel quarantine for foreign travellers.
India has ordered COVID-19 testing at airports from Wednesday for all arrivals from 'at-risk' countries.
U.S. President Joe Biden will provide fresh details of the variant and the response of the United States on Monday, the White House said.
Travel agents in Asia said some travellers were starting to consider cancelling or delaying trips, threatening the global tourism industry's already fragile recovery.
South Africa has denounced restrictions on travel from the region as unfair and potentially harmful to the economy, saying it was being punished for its scientific ability to identify variants early.
The WHO urged members to speed vaccination of high-priority groups and ensure efforts to keep up essential health services, advising a risk-based approach to adjusting global travel curbs.
"Omicron has an unprecedented number of spike mutations," it said, adding: "COVID-19 cases and infections are expected in vaccinated persons, albeit in a small and predictable proportion."
Scientists say Omicron has about double the number of mutations to the spike protein as the Delta variant that currently dominates.
The Philippines launched on Monday an ambitious drive to vaccinate nine million people against COVID-19 in three days, deploying security forces and thousands of volunteers in a programme made urgent by the emergence of the Omicron variant.
Britain, which said it would call an urgent meeting of G7 health ministers on Monday, will unveil new guidance later on extending the rollout of COVID-19 booster shots to the under-40s and possibly reducing the gap between second doses and boosters.
Oil prices and European stock markets were up on Monday, recovering partly from Friday's sell-off. Markets had been expecting governments and central banks around the world to start withdrawing some of the tens of trillions of dollars meant to keep businesses and households afloat during the pandemic. Another coronavirus wave could mean more support.
European Central Bank chief Christine Lagarde sought to reassure investors that the euro zone could cope with another wave of the pandemic.
"There is an obvious concern about the economic recovery in 2022, but I believe we have learnt a lot," she told Italian broadcaster RAI late on Sunday. "We now know our enemy and what measures to take. We are all better equipped to respond to a risk of a fifth wave or the Omicron variant."
With France also aiming to accelerate its vaccination campaign, President Emmanuel Macron tweeted that he had received a booster shot.
More than 261.17 million people in over 210 countries have been reported to be infected by the novel coronavirus since the first cases were identified in China in December 2019 and 5,456,515 have died, according to a Reuters tally.
(Reporting by Reuters bureaus Writing by Himani Sarkar and Catherine Evans Editing by Clarence Fernandez and Peter Graff) ((email@example.com))