The new omicron coronavirus variant kept spreading around the world on Sunday, with 13 cases found in the Netherlands and two each in Denmark and Australia, even as more countries tried to seal themselves off by imposing travel restrictions.

Dutch health authorities announced that the 13 cases of the variant were found among passengers who were on flights from South Africa that arrived in Amsterdam on Friday.

Authorities had tested all of the more than 600 passengers on those two flights and had found 61 coronavirus cases, going on to test those for the new variant.

“It is not unlikely more cases will appear in the Netherlands,” health minister Hugo de Jonge told a news conference in Rotterdam. “This could possibly be the tip of the iceberg.”

The variant has now been detected in South Africa, Britain, Germany, Italy, the Netherlands, Denmark, Belgium, Botswana, Israel, Australia and Hong Kong.

The discovery of omicron, dubbed a “variant of concern” last week by the World Health Organisation, has sparked worries around the world that it could resist vaccinations and prolong the nearly two-year Covid-19 pandemic.

Omicron is potentially more contagious than previous variants, although experts do not know yet if it will cause more or less severe Covid-19 compared to other strains.

A South African doctor who was one of the first to suspect a different coronavirus strain said on Sunday that symptoms of the omicron variant were so far mild and could be treated at home.

Angelique Coetzee, chair of the South African Medical Association, told Reuters that on November 18 she noticed seven patients at her clinic who had symptoms different from the dominant delta variant, albeit “very mild”.

Denmark’s infectious disease authority said on Sunday it had registered two cases of the variant in travellers from South Africa.

Health officials in Australia’s most populous state, New South Wales, said two passengers who arrived in Sydney from southern Africa on Saturday evening had tested positive for the variant.

Germany said on Sunday it had detected a total of three cases of the new Omicron coronavirus variant after a suspected infection in the western state of Hesse was confirmed.

Austria was investigating a suspected case on Sunday and in France health minister Olivier Veran said the variant was probably already circulating there.

Countries have imposed a wave of travel bans or curbs on southern Africa to try to stem the spread. On Sunday, most Gulf stock markets fell sharply in early trade, with the Saudi index suffering its biggest single-day fall in nearly two years.

In the most far-reaching effort to keep the variant at bay, Israel announced late on Saturday it would ban the entry of all foreigners and reintroduce counter-terrorism phone-tracking technology to contain the spread of the variant.

The top US infectious disease official, Anthony Fauci, said Americans should be prepared to fight the spread of the new variant, but it is too soon to say what actions are needed, including possible mandates or lockdowns.