With coronavirus cases increasing in its immediate vicinity in Alaska, the vast Denali National Park and Preserve reimposed a mask mandate on Friday for federal buildings and on park buses.

The National Park Service said the decision was made after community transmission of the coronavirus in Denali Borough, which includes the western part of the national park, reached a high level this week, as defined by guidance from the Centers for Disease Control and Prevention. The mask mandate will remain in place until coronavirus cases decline and the risk of transmission falls to medium, Sharon Stiteler, public affairs officer for Denali park and preserve, said on Friday.

The rule requires people ages 2 and older, regardless of vaccination status, to wear masks in Park Service buildings and spaces in the park, including visitor centers, offices, bookstores and bus depots. The policy also applies to all tour buses and shuttles, Ms. Stiteler said.

“According to frontline rangers, people are readily complying with our signs,” Ms. Stiteler said. “Our rangers keep an eye out for people who may not have a mask with them and offer one from our supply so they can come inside our visitor centers.”

As of Friday, an average of 28 virus cases per day were reported in Denali Borough, an increase of 63 percent from two weeks ago, according to a New York Times database.

The return of masking orders comes as the rapidly spreading Omicron subvariant known as BA.5 is now dominant among new U.S. cases, according to federal estimates, and national parks around the country are in their busy summer season and taking on more tourists and seasonal workers. Denali, home to North America’s tallest peak, is one of Alaska’s most popular parks.

The average number of new confirmed cases reported daily across the United States surged to record highs in January, driven by the Omicron variant. The surge receded swiftly as the winter ended, but the average started to rise again in the spring. In recent weeks, the number of new confirmed cases has been around 100,000 a day, according to a New York Times database.

Following President Biden’s executive order in January 2021 calling for federal agencies to follow C.D.C. guidance on masking and other measures in federal buildings and on federal lands, the Park Service, a branch of the Department of Interior, announced face coverings were mandatory in all Park Service buildings and facilities, as well as in outdoor spaces, like trails, if social distancing could not be maintained.

The Park Service updated its guidance in March 2022 after the C.D.C. announced its latest strategy, in which localities can determine masking policies based on the level of virus transmission in the area. At the time, Denali lifted its mask mandate in accordance with the C.D.C. guidance.