Britain and Denmark report a rise in known Omicron cases.

Britain and Denmark each reported a rise in confirmed coronavirus cases of the new Omicron variant on Sunday as countries with robust testing uncover more known instances of the variant in their backyards.

There were 86 new cases of the Omicron variant, Britain’s health security agency said on Sunday, bringing the total number of cases in the country to 246 — nearly double the total number of cases reported on Friday. In Denmark, local health authorities confirmed there were 183 confirmed cases of the variant, more than triple the total number of suspected cases reported on Friday.

Both Britain and Denmark are widely perceived as leaders in genomic sequencing and testing, giving them an edge over tracking the spread of the virus and its mutations. Still, much remains unknown about the Omicron variant.

The detection of more Omicron cases adds to a growing anxiety as overall coronavirus infections continue to increase in the region. Other European countries such as Austria and Germany recently implemented lockdowns or tightened rules for unvaccinated people to clamp down on the resurgence of cases. In Austria, the lockdown has prompted mass protests. France also imposed new border rules for incoming travelers.

On Sunday, officials in Britain said they were still gauging the potential impact of the new variant. But increasing the uptake of vaccines and boosters would be the “surest defense,” Dominic Raab, Britain’s deputy prime minister, said to the BBC.

“Our message is this: Enjoy Christmas this year. The vaccine rollout means we’re in a position to do so,” Mr. Raab said.

Britain reported an average of 44,385 daily cases last week, an 11 percent increase compared to two weeks ago. Deaths, in contrast, have decreased by 20 percent in that time frame.

Britain levied new restrictions last week to combat Omicron, including a requirement that all international travelers take a coronavirus test within 48 hours of departure, and an additional test within two days after arrival. The government also mandated the wearing of masks in indoor spaces like public transport and shopping centers.

Some critics and experts, fearing another spike in cases like that of last winter, which sent the nation into a monthslong national lockdown, said they worried that the new rules had been imposed too late.

And on Monday, the United States will begin requiring all incoming air travelers to show proof of a negative test taken the day before departure, regardless of their vaccination status or citizenship.