Announcing a public health emergency of worldwide concern, or PHEIC, grants the WHO director-general certain powers, including the ability to issue recommendations for how countries should respond.
Hundreds of people infected with the new virus have fallen ill in China, and 17 have died.
"It's too early to consider this as a public health emergency of worldwide concern", Didier Houssin, the chair of the emergency advisory committee, said, noting that the panel "was very divided, nearly 50-50". "But it has not yet become a global health emergency. It may yet become one", said Tedros.
Speaking earlier in the day, the head of Quebec's public health authority said five people in that province were under surveillance for possible exposure. Potential cases in other countries are still being investigated. Tedros accepted that suggestion, but China has to agree to it before a team can go. Here are some examples.
Health officials fear the transmission rate could accelerate as hundreds of millions of Chinese travel at home and overseas during week-long holidays for the Lunar New Year, which begins on Saturday.
"The mass involuntary quarantine in Wuhan and its neighboring cities is counterproductive", added Lawrence Gostin, a public health expert at Georgetown University Law School in Washington.
On the eve of the Lunar New Year, transportation was shut down Friday in at least 10 cities with a total of about 33 million people.
Wuhan, a city of 11 million where the virus was first identified, is in virtual lockdown.
There is no vaccine for the virus, which can spread through respiratory transmission.
"The perception of this declaration by the worldwide community, in the most affected country, by the people struggling with the virus, certainly has to be considered", he said. "It's a bit like a sledgehammer".
When asked if World Health Organization backed the quarantine measures, Tedros said China as a sovereign nation could take the steps it wanted to control the outbreak.
Discovery: Today, researchers led by Zheng-Li Shi at the Wuhan Institute for Virology, posted a paper describing the virus in detail for the first time, including the picture of the virus infecting cells. "Anything that limits people's travels during an outbreak would obviously work".
Avoid close contact with people who are sick. The Wuhan outbreak is suspected to have begun from wild animals sold at a food market in the city.
Chinese authorities on Thursday reported dozens of new infections, bringing the confirmed total to 571.
With the Chinese New Year on Saturday, January 25, there are concerns increased travel across China and high concentrations of people in public places will help the virus spread.
The source of the original outbreak was also traced to a seafood market in Wuhan City.
The limited cases of the coronavirus outside China mean that there is not enough evidence to determine whether it is a global emergency, according to the organization. The PHEIC designation (pronounced "fake") was created following an update to the International Health Regulations after that outbreak. Previous global emergencies have been declared for the emergence of Zika virus in the Americas, the swine flu pandemic, and polio.
There are two active PHEICs: the ongoing Ebola outbreak in the Democratic Republic of the Congo and the continued transmission of polio.
Helen Branswell contributed reporting.