Some Omicron cases are experiencing "mild" symptoms and experts should have more information about the transmission of the new COVID variant within days, an epidemiologist at the World Health Organization (WHO) has said.
This is faster than the "weeks" the WHO had predicted last week it would take to assess the data available on the strain after designating it a "variant of concern", its highest rating.
At least 23 countries, including the UK, have now reported Omicron cases and the WHO expects that number to rise.
Speaking at a news conference, Dr Maria van Kerkhove said one of the possible scenarios was that Omicron may become more transmissible than Delta but experts do not yet know about its severity.
She said the WHO had seen reports of Omicron case symptoms ranging from mild to severe disease.
"There is some indication that some of the patients are presenting mild disease," she told reporters.
And she said there was a suggestion of increased hospitalisations across South Africa, one of the first nations to detect it, but she pointed out that this could be down to more cases there.
The organisation said earlier this week preliminary evidence raised the possibility the COVID variant has mutations which could help it evade an immune-system response and boost its ability to spread from person to person.
It comes as a key group of UK scientists warned Omicron could trigger a surge in COVID infections bigger than previous waves in the country with a risk it may overwhelm the NHS.
Experts on the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG), which advises the government, held an extraordinary meeting last week to consider the new COVID variant, known scientifically as B.1.1.529, following its detection in South Africa.
According to a note of their meeting, which was observed by both a key Department of Health official and England's deputy chief medical officer Jonathan Van-Tam, the group concluded the introduction of Omicron to the UK "might have very serious consequences".
Nine more cases of Omicron have been detected in England, taking the total to 22, according to the UK Health Security Agency (UKHSA).
The people who have tested positive and their contacts are all isolating. Work is taking place to identify any links to travel to southern Africa.
Omicron cases have now been identified in the East Midlands, East of England, London, the South East and North West.
A further case has been detected in Scotland, bringing the total there to 10.
Meanwhile, Dr Mike Ryan, WHO's emergency director, said there was no evidence yet that giving booster jabs to the whole population, including healthy people, will provide greater protection against the disease, and said governments around the world should ensure all vulnerable individuals are offered at least one vaccine dose.
Prime Minister Boris Johnson has pledged to offer a booster COVID shot to all over-18s in the UK by the end of January.
But, responding to a question from Sky's Tom Clarke, Dr Ryan said: "Right now there is no evidence that would suggest that boosting the entire population is going to necessarily provide any greater protection for otherwise healthy individuals against hospitalisation or death.
"The real risk of severe disease, hospitalisation and death lies in particularly at-risk and vulnerable individuals who do require protection against all variants of COVID-19."
And his WHO colleague, Dr Soumya Swaminathan, said: "Our goal should be to protect those who have not received their primary course of vaccination."
The NERVTAG meeting, held via telephone conference on Thursday afternoon, came the day after South African authorities first reported the discovery of the Omicron variant.
The UK government added six southern African countries, including South Africa, to its travel red list on Friday. A further four countries were added over the weekend.
Other nations have also introduced travel bans, but the WHO said those restrictions are having an impact on global co-operation against Omicron by causing "challenges" to the sharing of laboratory samples from South Africa that can help get a better grip on the new variant.
WHO director-general Tedros Adhanom Ghebreyesus called for "tailored" intervention by countries, including testing travellers before and after they arrive in a country, and advised against "blanket travel bans" that "place a heavy burden on lives and livelihoods".
The high number of mutations on the protein spike that Omicron uses to infect human cells could mean existing vaccines need to be modified.
Some vaccine manufacturers have tried to calm fears about the new strain by predicting existing jabs would continue to prevent severe disease.