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 Organisation 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, Prof. 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 later, the White House said.