New COVID-19 cases increased 55% worldwide in the week ending Jan. 9, the World Health Organization said on Tuesday. 

More than 15 million new cases were reported, up from the about 9.5 million in the previous week, the United Nation's health agency said in its weekly epidemiological update. Omicron is "rapidly replacing" other variants of the virus, according to the update.

"In terms of disease severity, there is growing evidence that the Omicron variant is less severe as compared to other variants," the WHO said.

Last week's increase marked a slowdown from the 71% spike reported the previous week, following a "gradual" rise since October, according to the update. 

Reported cases in the Americas increased 78%, down from a 100% increase the week prior. The greatest increase in new cases came in Southeast Asia, where newly reported cases increased 418%, the WHO said.

On average, the U.S. is now reporting more than 750,000 new COVID-19 cases per day, a more than sixfold increase compared to one month ago, according to federal data. 

On Monday, the U.S. recorded a single-day high of more than 1.4 million new cases, following soaring demand for tests and a backlog of weekend reporting data. 

The surging national case numbers may not be indicative of what is happening in every region of the country and the overall increase in the U.S. does not necessarily mean that some regions will not see a plateau or decline in numbers. 

Although the incredibly high case totals are most certainly due, in large part, to the country's latest omicron surge, the data is incredibly volatile on a day-to-day level. Many states no longer report data over the weekends, which can create an illusion of very low totals on weekends and very high daily numbers on Mondays and Tuesdays. 

Also, there was a major surge in demand over the holidays, which could artificially increase numbers. However, experts say overall totals are likely undercounted given the increased use of at-home COVID-19 testing.

-ABC News' Arielle Mitropoulos

In Los Angeles, the hard-hit Providence Cedars-Sinai Tarzana Medical Center is "in complete disaster mode," Dr. Eric Snyder, medical director of the emergency department, told ABC News.

The emergency room is overflowing, with 22 patients currently waiting for beds.

"These are patients who’ve been accepted by a doctor, but have no place to go," he said.

Snyder said the hospital is in far worse shape than at any point in the pandemic. Tents have been set up behind the hospital and a doctor is going patient to patient, hoping to send those home who don't need hospital care, he said.

“We are 100% broken," Snyder said.

-ABC News' Matt Gutman

Massachusetts Gov. Charlie Baker said he's activating 500 more National Guard members to support the state's strained health care system.

Baker had activated 250 National Guard members when students returned to classrooms in the fall to help with school transportation, ABC Boston affiliate WCVB reported.

In Maine, Gov. Janet Mills is activating up to 169 National Guard members to help overrun hospitals, ABC affiliate WMTW reported. Maine hit a hospitalization record Monday with over 400 COVID-19 patients in hospitals, according to WMTW.