Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

The World Health Organization is not yet ready to declare the newly discovered coronavirus a global public health emergency, health officials said Wednesday, even as the death toll reached 17.

More than 500 cases of the respiratory illness have been reported in China, mostly in or near the city of Wuhan. The number is expected to grow by the hour, as cases are detected in other countries, including the United States.

"To proceed, we need more information. For that reason, I have decided to ask the emergency committee to meet again tomorrow," WHO Director-General Dr. Tedros Adhanom Ghebreyesus said during a news conference Wednesday.

On Tuesday, the Centers for Disease Control and Prevention confirmed the first case of the disease in the U.S., a man in his 30s in Washington state. The man became ill after visiting Wuhan. He is said to be in good condition, in isolation at a hospital in Everett.

The WHO's announcement comes as the city of Wuhan is preparing to quarantine its residents in an effort to control the outbreak. Beginning Thursday morning, all public transportation out of the Wuhan city limits will shut down.

No people in #Wuhan, C China's Hubei will be allowed to leave the city starting 10 a.m. of Jan. 23. Train stations and airport will shut down; the city bus, subway, ferry and long-distance shuttle bus will also be temporarily closed: local authority #WuhanPneumonia #coronavirus pic.twitter.com/8pohJ4Uixo — People's Daily, China (@PDChina) January 22, 2020

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

That will likely disrupt plans for millions of people in China, who are expected to travel for the Lunar New Year, a major holiday.

The global spread of the new coronavirus also comes during peak flu season in the Western Hemisphere. Symptoms of both respiratory viruses may mirror each other, potentially complicating the initial diagnosis.

"Many of these pathogens that are circulating are very difficult to distinguish," Maria Van Kerkhove, acting head of the WHO's emerging diseases division, said during Wednesday's news conference.

"What we understand from the 2019 novel coronavirus is that it can cause a range of disease in individuals who are infected from mild disease all the way to severe disease and death," she added.

The WHO has declared a global public health emergency just a handful of times in the recent past: the H1N1 (swine flu) pandemic in 2009; the 2014 outbreaks of polio in the Middle East and Ebola in West Africa; the 2015-16 Zika virus epidemic; and last year’s Ebola epidemic in Congo.

The declaration is meant, in part, to improve global communication about the new coronavirus, referred to as 2019-nCoV, and an illness that remains poorly understood.

"Part of what is driving the levels of concern that we're seeing globally is the lack of information," Lauren Sauer, director of operations for the Johns Hopkins Office of Critical Event Preparedness and Response, said.

"We need more epidemiological data to understand how this is spread, what the clinical illness looks like, how these patients should be identified and isolated," Sauer told NBC News on Wednesday.

Sauer added it's important for health care workers to have a good understanding of which patients need hospitalization and quarantine, and who do not.

"If we start hospitalizing everyone who potentially has a travel history to Wuhan, we can quickly overwhelm hospital systems globally," Sauer said.

Follow NBC HEALTH on Twitter & Facebook.