AMSTERDAM — The Netherlands' public health institute reported two local cases of the Omicron COVID-19 variant dating back to November 19 and 23.

It is not yet clear whether the people had travelled to South Africa, which alerted the global community about the new variant last week.

The cases predate those found in passengers that arrived in Amsterdam from South Africa on November 26

The World Health Organization has said the variant, named Omicron, poses a "very high" global risk due to a number of concerning mutations.

Many other European countries including Spain, Portugal, Denmark, Germany, the Czech Republic, Italy, and Belgium have now announced that they have detected cases of the new coronavirus strain.

On Monday afternoon a hospital in Madrid announced an Omicron case in a 51-year-old man who had returned to Spain from South Africa on Sunday and was showing mild symptoms.

Earlier Portugal said it had detected its first cases of the Omicron variant associated with players of the Belenenses SAD football club.

The country's public health agency said preliminary tests suggested that all 13 cases associated with players are related to the Omicron. One of the players had travelled to South Africa, where the variant was first detected.

The French health ministry said there were at least "eight possible cases" of the variant. Meanwhile, researchers said one case was reported on France's Indian Ocean island of La Réunion.

Speaking at a press conference on Sunday, European Commission President Ursula von der Leyen said a "race against time" was underway to tackle the Omicron variant.

The new variant poses a "very high" global risk, the World Health Organization said in a technical brief on how to prepare for Omicron.

The WHO said the variant had a high number of mutations, some of them concerning and pointing to "immune escape potential and higher transmissibility." That means it is uncertain if the current COVID-19 vaccines will work against it and that the variant has the potential to spread faster.

Experts in South Africa said the variant was likely behind a "rapid rise in cases" in the last two weeks in the country but that it was too early to tell if the variant was more severe.

WHO urged countries to "ensure mitigation plans are in place to maintain essential health services" in case of a potential hospitalisation surge.

The first case of Omicron was detected in South Africa on November 9. South Africa's health minister Dr Joe Phaahla condemned recent travel bans as "counterproductive" given that many countries have now reported cases.

The Dutch public health authority confirmed Sunday that 13 people who arrived in the Netherlands on flights from South Africa on Friday have so far tested positive for the new Omicron coronavirus variant.

The 61 people who tested positive for the virus on Friday after arriving on the last two flights to Amsterdam’s Schiphol airport before a flight ban was put in place were immediately put into isolation while sequencing was carried out to establish if they had the new variant.

The public health institute said in a statement that testing was continuing on the samples.

Most of the 61 people who tested positive were put into isolation at a hotel near the airport, while a small number were allowed to sit out their quarantine at home under strict conditions.

Health authorities appealed to all travellers who returned from southern Africa in the past week to get tested and set up a test centre at Schiphol Airport for Dutch citizens returning from the region. The tests are voluntary, and travellers can wait for the results in isolation at home.

Face masks are required from Tuesday in England's shops, public transport and other indoor settings over the emergence of a new COVID-19 variant.

Scotland reported three additional cases of the variant on Tuesday morning, bringing the total number in the UK to at least 14.

As part of the measures entering into force on Tuesday, international arrivals to the UK will also need to take a PCR test on the second day after arriving and self-isolate until they receive a negative result.

The infections of the new variant, which preliminary evidence suggests might be highly transmissible and more resistant to current treatment, including vaccines, prompted Prime Minister Boris Johnson to tighten entry requirements.

"We're not going to stop people travelling but will require anyone who enter the UK to take a PCR test by the end of the second day after their arrival and to self-isolate until they have a negative result," Johnson said during a press conference on Saturday afternoon.

The measure came into effect on Tuesday. At least 14 cases of the variant have been reported in the UK.

Malawi, Mozambique, Zambia and Angola have been added to the country's travel red list, alongside South Africa, Botswana, Eswatini, Lesotho, Namibia and Zimbabwe which were added earlier this week.

It means travellers from these countries must book a stay in a quarantine hotel.

Johnson also said that contacts of suspected Omicron cases will be asked to self-isolate for 10 days regardless of their vaccination status and that requirements on face coverings in shops and public transport will be tightened.

The "temporary and precautionary" new rules will be reviewed in three weeks.

Two confirmed Omicron infections were initially reported by the Bavarian Health Ministry on Saturday.

The two infected people returned from South Africa on November 24 and have been placed in self-isolation. The health authorities of Germany's southeastern lander called on passengers from the same flight to report to their local health authorities as soon as possible and for anyone who visited South Africa within the past two weeks to do a PCR test.

Italy's Higher Institute of Health (ISS) also announced on Saturday evening that a first case of the Omicron variant had been detected in Italy in a person who had recently been in Mozambique.

"The patient and his family members are in good health," it said in a statement.

The Czech Republic also detected a first case of the new strain on a COVID-19 patient. The woman is hospitalised with light symptoms in Liberec, in the north of the country.

They come a day after Belgium became the first European Union member state to report a case from a traveller returning from Egypt.

Dutch authorities are meanwhile sequencing samples from 61 passengers from two planes returning from South Africa on Friday and who tested positive for COVID-19.

EU member states on Friday closed their borders to travellers from seven Southern African countries — Botswana, Eswatini, Lesotho, Mozambique, Namibia, South Africa, and Zimbabwe — because of the variant.

The decision came hours before the World Health Organisation (WHO) gave the variant, first known as B.1.1.529, its name and labelled it a "variant of concern".

The United Nations' health agency stressed that "preliminary evidence suggests an increased risk of infection" with experts also worried that the variant's "large number of mutations" make it more immune to current treatment, including vaccines.

Other countries to have imposed travel restrictions with southern African countries include Canada, and the US.