Dutch health authorities said Saturday that 61 passengers aboard two KLM flights from South Africa tested positive for Covid-19 and the results were being examined for the new Omicron variant.
The positive cases were now being quarantined in a hotel near Amsterdam's Schiphol Airport, where the 600 people on the planes from Johannesburg spent hours waiting on Friday.
"We now know that 61 of the results were positive and 531 negative," the Dutch Health Authority (GGD) said in a statement.
"The positive test results will be examined as soon as possible to determine whether this concerns the new worrisome variant, which has since been given the name Omicron variant."
EU health authorities have said the new strain poses a "high to very high risk" to the continent.
All passengers who tested positive must stay in hotel quarantine for seven days if they show symptoms and for five days if they do not, the GGD said.
Passengers who tested negative, but are remaining in the Netherlands, are expected to isolate at home.
"We understand that people are frustrated by this," the statement added, "people have just made a long trip with the idea that they will shortly be home," it said.
"Instead just after landing, they are confronted with a situation we have never before experienced in the Netherlands, namely that people have to be tested at Schiphol and are forced to wait until they get a result."
Those who do not live in the Netherlands can "continue their journey", it said.
Dutch national carrier KLM said it would continue to operate flights from Johannesburg and Cape Town "in compliance with the stricter protocol."
This meant that entry was banned to all passengers except for Dutch and EU residents, who must show a negative PCR test and self-quarantine on arrival.
"KLM is taking the situation very seriously and will continue to prioritise the safety of passengers and crew," the airline said in a statement, adding "it will therefore impose strict on-board safety requirements for passengers and crew."
A KLM flight from Johannesburg was scheduled to land at around 1100 GMT, Schiphol said on its website.