The United States reported its first known case of community transmission of Omicron on Thursday hours before President Joe Biden was due to lay out his strategy to fight the coronavirus in the winter as the highly contagious variant spread across the globe.

With authorities around the world scrambling to contain the spread of Omicron, the public health department in the northern US state of Minnesota said a fully vaccinated adult male who had recently travelled to New York City had been infected.

The person told state health investigators he attended the Anime NYC 2021 convention at the Javits Center from November 19 to 21 and developed mild symptoms on November 22.

"We are aware of a case of the Omicron variant identified in Minnesota that is associated with travel to a conference in New York City, and we should assume there is community spread of the variant in our city," New York City Mayor Bill de Blasio said.

"We are working closely with the State and the CDC, as well as the Javits Center's event organizers, and our Test and Trace Corps will be contacting conference attendees," he said in a statement.

The Omicron variant could slow global economic growth by exacerbating supply chain problems and depressing demand, US Treasury Secretary Janet Yellen told the Reuters Next conference on Thursday.

"There's a lot of uncertainty, but it could cause significant problems. We're still evaluating that," she said.

Germany announced it would bar the unvaccinated from all but essential businesses such as grocery stores and pharmacies, while legislation to make vaccination mandatory will be drafted for early next year.

"We have understood that the situation is very serious," Chancellor Angela Merkel told a news conference.

A nationwide vaccination mandate could take effect from February 2022 after it is debated in the Bundestag and after guidance from Germany's Ethics Council, she said.

Eager to avoid derailing a fragile recovery of Europe's biggest economy, Germany kept businesses open to the almost 69% of the population that is fully vaccinated as well as those with proof of having recovered from the virus.

Biden was due to speak at the US National Institutes of Health (NIH) to announce steps including extending requirements for travellers to wear masks through mid-March.

By early next week the United States will require inbound international travellers to be tested for Covid-19 within a day of departure, regardless of vaccination status.

And private health insurance companies will be required to reimburse customers for at-home Covid-19 tests, as part of a winter strategy that Biden is due to announce at 1840 GMT.

"The president is going to unveil a very robust plan, pull out all the stops to prepare for the winter and to prepare for the new variant," White House Covid-19 response coordinator Jeff Zients told broadcaster MSNBC.

Much remains unknown

Much remains unknown about Omicron, which was first detected in southern Africa last month and has been spotted in at least two dozen countries, just as parts of Europe were already grappling with a wave of infections of the Delta variant.

The European Union's public health agency said the variant could be responsible for more than half of all Covid infections in Europe within a few months, lending weight to preliminary information about its high transmissibility.

South Africa said it was seeing an increase in Covid-19 reinfections in patients contracting Omicron - with people who have already had the illness getting infected again - in a way that it did not see with other variants.

In the Netherlands, health authorities called for pre-flight Covid-19 tests for all travel from outside the European Union, after it turned out that most of the passengers who tested positive after arriving on two flights from South Africa on Nov. 26 had been vaccinated.

The first known US case, announced on Wednesday, was a fully vaccinated person in California who had travelled to South Africa. Two French cases, in the greater Paris region and in eastern France, were passengers arriving respectively from Nigeria and South Africa.

Travel restrictions

Russia has imposed a two-week quarantine for travellers from some African countries including South Africa, the Interfax news agency said, quoting a senior official. Hong Kong extended a travel ban to more countries and Norway, among others, re-introduced travel restrictions.

Amid all the new restrictions, Europe's largest budget airline, Ryanair, said it expected a challenging time at Christmas, although it was still optimistic about summer demand.

In France, the country's top scientific adviser, Jean-Francois Delfraissy, said the "true enemy" for now was still the more familiar Delta variant of the virus, spreading in a fifth wave.