The World Health Organisation says it's not yet clear whether Omicron, first detected in Southern Africa, is more transmissible than other variants, or if it causes more severe disease.
"Preliminary data suggests that there are increasing rates of hospitalisation in South Africa, but this may be due to increasing overall numbers of people becoming infected, rather than a result of specific infection," WHO said on Sunday.
It said understanding the level of severity of Omicron "will take days to several weeks".
The detection of Omicron triggered global alarm as governments around the world scrambled to impose new travel curbs and financial markets sold off, fearing the variant could resist vaccinations and upend a nascent economic reopening after a two-year global pandemic.
The WHO said it was working with technical experts to understand the potential impact of the variant on existing countermeasures against COVID-19, including vaccines.
Britain said it would convene an urgent meeting of G7 health ministers on Monday.
Dutch health authorities said 13 Omicron cases were found among people on two flights that arrived in Amsterdam from South Africa on Friday.
Authorities had tested all of the more than 600 passengers on the flights and found 61 coronavirus cases, going on to test those for Omicron.
"This could possibly be the tip of the iceberg," Health Minister Hugo de Jonge told reporters.
Omicron, dubbed a "variant of concern" last week by the WHO that is potentially more contagious than previous variants, has been detected in Australia, Belgium, Botswana, Britain, Denmark, Germany, Hong Kong, Israel, Italy, the Netherlands, France, Canada and South Africa.
Many countries have imposed travel bans or curbs on southern Africa to try to stem the spread. Financial markets dived on Friday and oil prices tumbled.
A South African doctor who was one of the first to suspect a different coronavirus variant said on Sunday that Omicron's symptoms were so far mild and could be treated at home.
Dr Angelique Coetzee, chair of South African Medical Association, told Reuters that unlike with Delta, patients had not reported loss of smell or taste and there had been no major drop in oxygen levels with the new variant.
In the most far-reaching restriction, Israel announced it would ban the entry of all foreigners for 14 days and reintroduce counter-terrorism phone-tracking technology to contain the variant's spread.
The top US infectious disease official, Dr Anthony Fauci, told President Joe Biden on Sunday it would take about two weeks to have more definitive information about Omicron.
The White House said Fauci believed existing vaccines "are likely to provide a degree of protection against severe cases of COVID".
Biden will give an update on the new variant and the US response on Monday.
The British government announced measures including stricter testing rules for people arriving in the country and requiring mask wearing in some settings.
More countries announced new travel curbs on southern African nations, including Indonesia and Saudi Arabia.
South Africa has denounced the measures as unfair and potentially harmful to its economy, saying it is being punished for its scientific ability to identify coronavirus variants early.
President Cyril Ramaphosa said his government was considering imposing compulsory COVID-19 shots for people in certain places and activities, and he slammed rich Western countries for what he called their knee-jerk travel bans.
Omicron has emerged as many countries in Europe are already battling a surge in COVID-19 infections.
The variant has also thrown a spotlight on huge disparities in global vaccination rates. Even as many developed countries are giving third-dose boosters, less than seven per cent of people in poorer countries have received their first COVID-19 shot, according to medical and human rights groups.
