The Omicron variant of coronavirus carries a very high global risk of surges, the WHO warned on Monday, as more countries reported cases, prompting border closures and reviving worries about the economic recovery from a two-year pandemic.

Scientists have said it could take weeks to understand the severity of Omicron, which was first identified in southern Africa. Its emergence has caused a strong global reaction, with countries imposing travel curbs and other restrictions, worried that it could spread fast even in vaccinated populations.

Spooked investors wiped roughly $2 trillion off global stocks on Friday. Financial markets were calmer on Monday, even after Japan, the world's third-largest economy, said it would close its borders to foreigners.

The World Health Organization advised its 194 member nations that any surge in infections could have "severe consequences" but said no deaths linked to the Omicron variant had been reported so far.

WHO director-general Tedros Adhanom Ghebreyesus said Omicron's emergence showed how "perilous and precarious" the situation was and called on health ministers meeting in Geneva to pursue a new accord on pandemics.

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 ban on arrivals by foreigners 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.

In Israel, a ban on arrivals by foreigners took effect overnight.

US President Joe Biden will provide fresh details of the variant and the US response 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, saying it is being punished for its scientific ability to identify variants early.

Scientist Richard Hatchett said Omicron's emergence had fulfilled predictions that transmission of the virus in low-vaccination areas would speed its evolution. Botswana and South Africa have fully vaccinated less than 25 percent of their populations.

"The inequity that has characterized the global response has now come home to roost," Hatchett, who is CEO of the Coalition for Epidemic Preparedness Innovations (CEPI), a foundation that funds vaccine development, told the Geneva meeting.

Vaccination drive

With the WHO urging members to speed up vaccinations of high-priority groups, the Philippines launched an ambitious drive to inoculate nine million people against Covid-19 in three days, deploying security forces and thousands of volunteers.

"Omicron has an unprecedented number of spike mutations, some of which are concerning for their potential impact on the trajectory of the pandemic," the WHO said.

Britain, which said it would call an urgent meeting of G7 health ministers on Monday, will unveil new guidance later on giving booster shots to under-40s.

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 coronavirus since the first cases were identified in China in December 2019 and 5,456,515​ have died, according to a Reuters tally.

Markets had been expecting governments and central banks around the world to start withdrawing some of the trillions of dollars meant to keep businesses and households afloat during the pandemic. Another coronavirus wave could mean more support.

Battered stocks and oil prices recovered partly from Friday's sell-off as markets latched onto hopes that Omicron would prove milder than initially feared.

European Central Bank President Christine Lagarde sought to reassure investors that the euro zone could cope with a resurgence 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."