The new coronavirus variant Omicron has been detected in 13 people who arrived in the Dutch capital Amsterdam on two flights from South Africa.
They are among 61 passengers who tested positive for coronavirus.
It comes as tighter restrictions come into force in the Netherlands, amid record Covid cases and concerns over the new variant.
This includes early closing times for hospitality and cultural venues, and limits on home gatherings.
Omicron was first reported to the World Health Organization (WHO) by South Africa on Wednesday, and early evidence suggests it has a higher re-infection risk. It has been categorised by the WHO as a “variant of concern”.
The Dutch National Institute for Public Health announced the 13 Omicron cases on Sunday, but noted that its investigation had “not yet been completed”, meaning the new variant could still be found in more test samples.
Dutch Health Minister Hugo de Jonge made an “urgent request” for people who have returned from southern Africa to get tested for Covid “as soon as possible”.
“It is not unthinkable that there are more cases in the Netherlands,” he told reporters.
Cases of the heavily mutated variant have now been reported in a number of countries around the world, including several in Europe, such as the UK, Germany and Italy.
The flights by Dutch national carrier KLM from Johannesburg arrived on Friday morning. The 600 passengers on board were held for several hours after arriving while they were tested for the virus.
New York Times correspondent Stephanie Nolen, who was on one of the flights, tweeted that the passengers were not even brought water while they remained on the plane.
Passengers travelling from South Africa to the UK via Amsterdam told the BBC that they were held on the tarmac at Schiphol airport for four hours, before eventually disembarking.
The passengers who were confirmed to have Covid-19 have since been quarantined at a hotel near Schiphol airport.
Those that tested negative have been asked to isolate at home for five days and take further tests. Officials said those in transit would be allowed to continue their journeys, though there were reports on Saturday that some passengers had not received written proof of a negative test and were therefore unable to board onward flights.
Dutch health authorities were also trying to contact thousands of other passengers who had travelled from seven southern African countries – South Africa, Botswana, Eswatini, Lesotho, Mozambique, Namibia or Zimbabwe – since Monday.
The Netherlands is one of many countries around the world to have now imposed travel restrictions on countries in southern Africa in response to the new variant.
Some have also gone further, with Israel set to bar entry to all foreign nationals. Switzerland has ordered that passengers from other countries where the variant has been detected, including the UK and the Netherlands, provide proof of a negative coronavirus test and quarantine for 10 days.
Meanwhile, the Netherlands is one of several European countries struggling to contain record numbers of infections.
It has a shortage of intensive care beds, operations are being cancelled and hundreds of people are dying of Covid every week, the BBC’s Anna Holligan in The Hague says.
An extended partial lockdown came into force on Sunday morning and will remain in place for at least the next three weeks.
Bars, restaurants and shops – whose opening hours have already been restricted for several weeks – are required to close earlier than before, and there is a limit on the number of guests allowed in homes.
People are also being encouraged to work from home where possible, but nurseries, schools and universities across the country remain open.
Thousands of people protested after the measures were announced.
The Netherlands has had nearly 20,000 confirmed Covid-related deaths since the pandemic started.