Dr. Anthony Fauci of the National Institutes of Health told CNBC on Monday that a quarter of China's coronavirus cases require intensive treatment.

"About 25% of them have very serious disease, requiring relatively intensive or really intensive care," said the director of the NIH's National Institute of Allergy and Infectious Diseases.

The NIH is part of the U.S. Department of Health and Human Services.

China's National Health Commission said Monday that the confirmed coronavirus cases in China increased to 17,205. The death toll rose to 361 there — with one additional fatality in the Philippines, the first outside China.

The World Health Organization said there's been more than 150 coronavirus cases in about two dozen countries outside of China. Last week, WHO declared the virus a global health emergency.

"It's escalating," Fauci said on "Squawk on the Street." "The number of cases that increase from one day to another is clearly going up in a very steep slope."

There are many health-care professionals and analysts, including Fauci, who believe the number of coronavirus cases to be much higher.

"There are probably a lot more people who were infected in China who have not been really counted ... because they were either asymptomatic or their symptoms were so light that they didn't come to the attention of health authorities," Fauci said. "The number is probably much larger."