Ghebreyesus praised the Chinese government for its cooperation and transparency on Thursday.

Advisers to the US Centers for Disease Control and Prevention had previously told CNN they were concerned that Chinese health officials had still not released basic epidemiological data about the Wuhan coronavirus outbreak, making it more difficult to contain.

WHO defines a public health emergency of international concern as "an extraordinary event" that constitutes a "public health risk to other States through the international spread of disease" and "to potentially require a coordinated international response." Previous emergencies have included Ebola, Zika and H1N1.

Ghebreyesus said that WHO's decision "should not be taken as a sign that WHO does not think the situation is serious, or that we are not taking it seriously."

WHO has played a number of roles in the international response to the outbreak, including coordinating with international authorities and researchers, as well as developing guidance for lab testing, treatment and prevention measures.

WHO has previously advised that people in the region avoid unnecessary contact with animals or animal products in markets, that they practice proper hygiene, and that they avoid eating raw or undercooked meat. People with "underlying medical conditions" should avoid live animal markets and raw meats altogether, as those people are "considered at higher risk of severe disease," according to the WHO.

Ghebreyesus said Thursday WHO does not recommend broadening restrictions on travel or trade at the moment, but does recommend ways to detect potential cases in airports and health care settings.

Hundreds of confirmed cases

The newly identified virus has infected hundreds of people worldwide and led to at least 17 deaths since the outbreak started in December. The majority of confirmed cases are in the Chinese province of Hubei -- where Wuhan is located -- with other suspected and confirmed infections cropping up in multiple countries and territories worldwide.

Scientists at Imperial College London say there could be far more cases. According to their estimates Wednesday, around 4,000 people are likely to have been infected by the new coronavirus in Wuhan alone as of January 18.

The United States announced its first case Tuesday, and is planning to expand health screenings at airports. Similar measures are being adopted by airports across Asia, including temperature screening of incoming passengers.

On Thursday, the US Centers for Disease Control and Prevention raised its travel notice for Wuhan to its highest level, "warning," recommending "that travelers avoid all nonessential travel to Wuhan, China."

Earlier in the week, the agency also activated its Emergency Operations Center -- as it has done with Ebola and vaping-related lung injuries -- to centralize the US response to the outbreak.

Signs and symptoms

According to health authorities, patients infected with the Wuhan coronavirus may have common symptoms such as fever, cough, headache or muscle pain. A subset of patients may have more severe illness including trouble breathing and scans revealing signs of infection in both lungs.

According to the CDC, "preliminary information suggests that older adults and people with underlying health conditions may be at increased risk for severe disease from this virus."

The CDC also says it believes symptoms may appear anywhere from two to 14 days after being exposed to the virus, based on what it knows about other viruses in the same family. Still, health experts say there's much to unravel about how -- and how quickly -- the virus spreads.

"At this time, it's unclear how easily or sustainably this virus is spreading between people," the CDC says.

The Wuhan coronavirus is in the same family as severe acute respiratory syndrome, or SARS -- which killed more than 700 people in 2002 and 2003 -- and Middle East respiratory syndrome, or MERS. The coronavirus family is also associated with the common cold.

Containing the outbreak

Chinese officials are racing to contain the outbreak after it was confirmed the infection can be spread from person to person, raising the possibility of increased transmissions as China enters into the busy Lunar New Year travel period.

Wuhan municipal government published a notice making it mandatory to wear face masks in public places on Wednesday.

"People who don't obey the requirements shall be dealt with by authorities in accordance with their respective duties and laws," the notice reads.

The city has taken a number of other measures, including the cancellation of upcoming Lunar New Year celebrations and putting the city on a partial lockdown on Thursday, halting city buses, ferries and subways, as well as airport and railway stations, according to Wuhan authorities.

Wuhan is the largest city in central China and a major transportation hub. Officials in China have linked the initial infections to a Wuhan seafood and wildlife market, which has been closed since January 1 to prevent further spread of the illness.