Two planes carrying some 600 passengers from South Africa landed in the Netherlands with 61 people infected with the coronavirus — including some cases believed to be the new omicron variant — Dutch health authorities said Saturday, after halting flights from several southern African countries over fears of the new omicron variant of the virus.

Passengers were tested at the airport, and those with infections will be isolated at a hotel, a regional Dutch health agency said.

“In a number of the tested persons the omicron variant is presumed to have been found,” the Netherlands Institute for Health said in a statement. An institute spokesperson said it was “almost certain” the cases were of the omicron variant, but that only additional testing would allow for complete certainty, Reuters reported.

Omicron, a variant of the novel coronavirus that some scientists fear could be more transmissible than the delta variant, was labeled a “variant of concern” by the World Health Organization on Friday. It was first detected in the southern region of Africa, where cases have started rising again in recent days.

A new round of countries on Saturday rushed to ban or curb travel from there in a bid to stem the spread of the just-discovered variant, which has raised concern about the global trajectory of the pandemic. The United States, the European Union and Britain swiftly imposed travel restrictions on South Africa and its neighbors, including Zimbabwe and Botswana, though the variant has also been identified in Belgium, Hong Kong and Israel.

The United States and several other countries imposed travel restrictions on South Africa and other African countries after the omicron variant was discovered. (Amber Ferguson/The Washington Post)

Amsterdam’s Schiphol Airport, where the two Friday flights landed, saw over 71 million passengers in 2019, making it one of the busiest and most connected travel hubs in the world.

Despite a mask mandate by the Dutch airline KLM, which operated the flights from South Africa, many passengers did not wear face coverings, said New York Times reporter Stephanie Nolen, who was aboard one of the planes. Passengers were stuck on the airport tarmac for about four hours before being sent to be swabbed, tweeted Nolen, who later said she tested negative.

A KLM spokesperson told Reuters that passengers had shown proof of vaccination or a preflight negative test before boarding flights in Cape Town or Johannesburg.

“It goes too far to say we are surprised” by the high number of cases, the airline spokesperson said. “But we don’t have an explanation.”

News of the omicron variant emerged as the Dutch government was preparing to begin stricter partial lockdown measures starting this weekend. On Thursday, the country of some 17.5 million reported a seven-day rolling average of more than 20,000 infections.

Starting Sunday, many businesses have been ordered closed between 5 p.m. and 5 a.m., a move that Dutch Prime Minister Mark Rutte said means the country will be “effectively closed” from the evening through the early morning.

The Dutch government was the first in Western Europe to return to a partial lockdown when it announced restrictions on shops and restaurants earlier this month. Rutte said Friday that his government was tightening measures because there had been minimal change in people’s behavior, citing traffic data.

A dozen protesters demonstrated peacefully in The Hague after Friday’s announcement, Agence France-Presse reported. But other recent anti-lockdown rallies in the Netherlands involved what the mayor of Rotterdam called an “orgy of violence,” with protesters throwing stones and police firing warning shots.

About 74 percent of the Dutch population has been fully vaccinated, and booster shots are being offered to vulnerable people, including the elderly and care workers.

Ellen Francis contributed to this report.

Read more:

A new coronavirus variant was detected in southern Africa on Nov. 25. Here’s what we know about it. (Adriana Usero/The Washington Post)