New York City reinstates mask advisory 'at all times' while indoors ahead of omicron detection

Officials in New York City have reinstated the city's mask advisory while indoors as the omicron variant continues to spread around the globe.

City health officials are "strongly recommending" that all residents wear masks inside public settings such as grocery stores, building lobbies and offices, regardless of vaccination status.

"This includes those who are vaccinated and those who have had COVID-19," Dr. Dave Chokshi, commissioner of the city's Department of Health and Mental Hygiene, told reporters at a news conference Monday. "Higher quality masks can offer additional protection, and masks are still required for everyone in public transit, health care settings, schools and congregate settings."

While the new variant has not been detected yet in New York City, there will likely be a positive case confirmed within days "based on what we know about its global spread," Chokshi said.

The World Health Organization warned in a report Sunday that omicron poses a high likelihood of further transmission with a “very high” global risk.

New Yorkers who traveled or gathered in groups over the Thanksgiving holiday are also encouraged to get tested, officials said.

About 88% of adults and 81% of kids ages 12 to 17 in New York City have received at least one dose of the vaccine, ABC New York station WABC reported. The average daily number of infections is 905, and that number is increasing, according to city health data.

Health officials are "very, very carefully" monitoring the variant, Mayor Bill de Blasio told reporters Monday. Nearly 5,400 people in the state are testing positive daily, on average, marking the highest number of new positive daily cases since February, according to the station.

President Joe Biden addressed the nation Monday, saying that while omicron is a "cause for concern," there is no need to panic. Omicron will come to the U.S. "sooner or later," Biden said.

Biden urged Americans to get vaccinated, including those who are now eligible for the booster, and to continue to wear masks indoors.

"We'll fight this variant with scientific and knowledgeable actions and speed -- not chaos and confusion," Biden said. "We have more tools today to fight the variant than we ever had before, from vaccines to boosters to vaccines for children."

Once omicron gets to the U.S., it will likely "spread widely," Dr. Anthony Fauci, the U.S. government's top infectious diseases expert, told ABC News' George Stephanopoulos in an interview Monday on "Good Morning America."

"It's inevitable that, sooner or later, it's going to spread widely because it has at least the molecular characteristics of being highly transmissible," Fauci said.

ABC News' Aaron Katersky, Brian Hartman and Arielle Mitropoulos contributed to this report.