Even more countries imposed travel restrictions trying to seal themselves off as the Omicron coronavirus variant kept spreading around the world on Sunday, with 13 cases found in the Netherlands and two each in Denmark and Australia. Dutch health authorities said the 13 cases of the variant were found among people on two flights from South Africa to 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 discovery of Omicron, dubbed a “variant of concern” last week by the World Health Organization, has caused worry around the world that it could resist vaccinations and prolong the nearly two-year COVID-19 pandemic. First discovered in South Africa, it has now been detected in Britain, Germany, Italy, the Netherlands, Denmark, Belgium, Botswana, Israel, Australia and Hong Kong. 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. Many countries have imposed travel bans or curbs on southern Africa to try to stem the spread. Financial markets dived on Friday as investors worried that the variant could stall a global recovery. Oil prices tumbled by about $10 a barrel. Most Gulf stock markets ended lower on Sunday, with the Saudi and Dubai indexes suffering their biggest single-day fall in nearly two years.