The COVID-19 omicron variant has an "unprecedented number of spike mutations" and poses a "very high" global risk, the World Health Organization said Monday.

Because of fears that the new variant has the potential to be more resistant to the protection offered by vaccines, there are growing concerns around the world that the pandemic and associated lockdown restrictions will persist for far longer than hoped.

"The overall global risk related to the new variant of concern omicron is assessed as very high," the WHO said, according to Reuters. The health body said countries should make sure that there are "mitigation plans in place" to deal with an increase in new cases. Scientists are working to determine how effective vaccines are in preventing infections with the new variant.

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The U.K. on Saturday tightened its rules on mask-wearing and on testing of international arrivals after finding two cases. New cases were confirmed Saturday in Germany and Italy, with Belgium, Israel and Hong Kong also reporting that the variant has been found in travelers.

A group of women show their COVID passports as they enter La Belle Angele nightclub on Nov. 25, 2021, in Edinburgh, Scotland.

A group of women show their COVID passports as they enter La Belle Angele nightclub on Nov. 25, 2021, in Edinburgh, Scotland. (Photo by Peter Summers/Getty Images) (Getty)

On Monday, Scotland reported six cases of the variant and have started contact tracing in order to "establish the origin of the virus and any further individuals they may have come into contact with in recent weeks," Humza Yousaf, the Scottish health secretary said, according to Reuters. Portugal also recorded 13 cases of the new variant.

A passenger wearing personal protective equipment queues to check-in for a flight at the Kuala Lumpur International Airport on Nov. 29, 2021, as countries across the globe shut borders and renewed travel curbs in response to the spread of a new, heavily mutated COVID-19 coronavirus variation.

A passenger wearing personal protective equipment queues to check-in for a flight at the Kuala Lumpur International Airport on Nov. 29, 2021, as countries across the globe shut borders and renewed travel curbs in response to the spread of a new, heavily mutated COVID-19 coronavirus variation. (Photo by Mohd RASFAN / AFP)  (Getty)

Dr. Anthony Fauci, the U.S.' top infectious diseases expert, said Sunday that he would not be surprised if the omicron variant was already in the United States, too.

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"We have not detected it yet, but when you have a virus that is showing this degree of transmissibility ... it almost invariably is ultimately going to go essentially all over," Fauci said on NBC television.

The variant has a high number of spike mutations that could affect transmissibility and immune response, said Ravindra Gupta, a professor of clinical microbiology at the University of Cambridge.

The WHO's warning comes a day after Dr. Angelique Coetzee, a board member of the South African Medical Association, who first alerted authorities to the presence of the omicron variant, said it presents "unusual but mild" symptoms. 

"Their symptoms were so different and so mild from those I had treated before," Coetzee told The Telegraph

Fox News' Peter Aiken and the Associated Press contributed to this report