Dutch health authorities said on Sunday they had found at least 13 cases of the new Omicron strain among 61 quarantined passengers who tested positive for coronavirus after arriving from South Africa.
"The Omicron variant has so far been identified in 13 of the positive tests. The investigation has not yet been completed. The new variant may be found in more test samples," the National Institute for Public Health (RIVM) said in a statement.
Dutch authorities began testing for the Omicron variant after 61 out of 600 passengers on board two flights from South Africa to Amsterdam on Friday tested positive for Covid-19.
Health Minister Hugo de Jonge made an "urgent request" to people returning from southern Africa to get tested for Covid "as soon as possible".
"It is not unthinkable that there are more cases in the Netherlands," De Jonge told reporters.
"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, and keep up with the sequencing" of Covid tests to look for the new variant.
The positive cases are being kept in quarantine at a hotel near Amsterdam's Schiphol Airport.