ONDON/AMSTERDAM — The new Omicron coronavirus variant kept spreading around the world on Sunday, with 13 cases found in the Netherlands and two in 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.

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

First discovered in South Africa, the variant has since been detected in Britain, Germany, Italy, Belgium, Botswana, Israel, and Hong Kong.

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 Omicron variant of the coronavirus.

Both people were asymptomatic, fully vaccinated, and in quarantine, NSW Health said. Another 12 passengers from southern Africa were also in 14 days of hotel quarantine, while around 260 other passengers and aircrew have been directed to isolate.