Dutch health authorities said on Sunday they had discovered at least 13 cases of the new Omicron strain among passengers on two flights from South Africa.
The infections were found after 61 out of 600 travellers on the two KLM planes tested positive for Covid-19 on arrival at Amsterdam's Schiphol Airport on Friday.
It was "not unthinkable" that there were further cases of the infectious Omicron variant in the Netherlands, Health Minister Hugo de Jonge said.
"The Omicron variant has so far been identified in 13 of the positive tests. The investigation has not yet been completed. the National Institute for Public Health (RIVM) said in a statement.
"The new variant may be found in more test samples."
The 61 Covid-positive passengers are almost all in quarantine at a hotel near Schiphol Airport, while a few have been allowed to go into home quarantine.
Passengers who tested negative must also go into isolation at home if remaining in the Netherlands, but others were allowed to continue with their journeys.
"We will control whether they keep to those rules," de Jonge told reporters.
The Netherlands is now stepping up testing for travellers from South Africa, Botswana, Malawi, Lesotho, Swaziland, Namibia, Mozambique and Zimbabwe, in the region where the new variant was first detected.
"We are making an urgent request to people coming from this area to get tested as soon as possible," de Jonge said.
- 'We are concerned' -
The new cases raise fears that the variant has spread further in the Netherlands, the latest in a series of European countries including Germany, Britain and Belgium where Omicron been detected.
"It is not unthinkable that there are more cases in the Netherlands," De Jonge said. "Yes, we are concerned. But how much we don't know yet."
The minister added: "What is important now is that we keep our finger on the pulse."
Tighter, pre-announced Covid restrictions came into force on Sunday in the Netherlands with bars, restaurants and most shops closing at 5:00 pm.
Some passengers on the two South African flights recounted an hours-long ordeal while they waited to be tested, with one describing it as "Dystopia Central Airline Hallway".
New York Times global health reporter Stephanie Nolen said passengers, including babies and toddlers, were crammed together waiting to get tested, while "still 30 percent of people are wearing no mask or only over mouth".
Both flights were already in the air by the time Dutch authorities announced a ban on non-Dutch and -EU nationals travelling to the country on Friday.
Dutch media said vaccinated people had not been required to take CPR Covid tests before boarding.
A relieved group of 161 passengers made it back from South Africa to the Netherlands on a flight from Johannesburg on Saturday under tougher restrictions, while 107 passengers were stopped from boarding.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)