Experts say the steady surge of infections is expected to only intensify in the weeks to come, after millions of Americans traveled and gathered over the Thanksgiving holiday.

Nationally, hospitalization numbers have ticked up to about 53,000, according to federal data. After nearly 10 weeks of steady declines, this marks the third consecutive week that the U.S. has seen an increase in hospitalizations.

Older populations are bearing the brunt of this latest surge, with Americans 65 and older accounting for more than 41% of current hospitalizations.

Minnesota and Michigan currently hold the nation's highest case rate, followed by Wisconsin, North Dakota, New Hampshire, Arizona, New York, Pennsylvania and Ohio, according to federal data.

-ABC News' Arielle Mitropoulos

ABC News' chief medical correspondent Dr. Jennifer Ashton said Monday that she "absolutely" believes omicron is already circulating in the U.S.

"When you hear the virus has been detected in so many countries, it should come as no surprise it's here," she said. "Viruses mutate for a living. As long as there are unvaccinated people in the world -- in South Africa, 6% vaccination rate -- this should not be a surprise."

Ashton said it will be critical for the U.S. to ramp up its genetic sequencing to effectively monitor the spread of new variants, including omicron.

Ashton, however, stressed that the U.S. in a much better place than one year ago.

"We're better at testing, we're better at surveillance, we're better at treating and we're better at preventing," Ashton said.

-ABC News' Arielle Mitropoulos

President Joe Biden stressed Monday that the omicron variant is a "cause for concern, not a cause for panic."

"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."

If updated vaccines are needed to fight omicron, "we will accelerate their development and deployment with every available tool," Biden said.

"I want to reiterate Dr. [Anthony] Fauci believes that the current vaccines provide at least some protection" against omicron, "and the booster strengthens that protection significantly," Biden said.

"We do not yet believe that additional measures will be needed," Biden said, but his administration is working with Pfizer, Moderna and Johnson & Johnson to develop plans in case.

"Sooner or later" omicron will be in the U.S., the president said.

Biden said his team will release a detailed strategy outlining the fight against COVID-19 this winter. He said there won't be lockdowns and instead is promoting vaccinations, boosters and testing.

The president said "we need to do more than vaccinate Americans," adding, "We have to vaccinate the world, as well."

"We ship for free, more vaccines to other countries than all other countries in the world combined. ... Now, we need the rest of the world to step up," he said.

Biden again emphasized that the best protection is getting vaccinated and urged any adults who were fully vaccinated before June 1 to go get a booster immediately. He also asked Americans to wear masks indoors.

New York City officials reinstated a mask advisory on Monday, “strongly recommending” all residents, regardless of vaccination status, wear masks inside public settings.

Public settings include grocery stores, building lobbies and offices, said Dr. Dave Chokshi, commissioner of the city's Department of Health and Mental Hygiene.

"Masks are still required for everyone in public transit, health care settings, schools and congregate settings," he added.

The omicron variant will likely be detected in New York City in the coming days, Chokshi said.

Health officials are "very, very carefully" monitoring the variant, said Mayor Bill de Blasio.

-ABC News' Aaron Katersky, Brian Hartman, Arielle Mitropoulos