The WHO's emergency committee, a group of infectious-disease experts, last week delayed a decision on whether to make the emergency declaration.
The World Health Organization defines a global emergency as an extraordinary event that constitutes a risk to other countries and requires a coordinated worldwide response.
"There's reasonable probability that we won't be able to contain this virus in the foreseeable future", Gostin said.
The effects of the virus are already weighing heavily on China's economy, the world's second-biggest, with companies cutting corporate travel and tourists cancelling trips during the traditional Lunar New Year holiday period.
The vast majority - all but 68 cases - have been recorded on mainland China, making it more widespread in the country than the SARS outbreak in 2002 and 2003, The New York Times reported. He had underlying health conditions prior to contracting coronavirus.
China reported its biggest single-day jump in novel coronavirus deaths on Thursday, as confirmation that three Japanese people evacuated from Wuhan, the outbreak's epicentre, were infected deepened fears about a global contagion, and more countries announced plans to bring their citizens home from the hard-hit city. This is the world's eighth instance of human-to-human spread outside of China.
Various airlines are cutting flights to the country, from British Airways and Lufthansa to Air Canada and American Airlines.
Tedros also called on countries to combat false information about the virus circulating on social media, including Twitter.
Chinese authorities have taken extraordinary steps to arrest the virus's spread, including effectively locking down more than 50 million people in Wuhan and surrounding Hubei province. About 25 percent of those infected get very ill, and about 2 percent have died, primarily older people. A week ago, the organization said the outbreak wasn't yet an global emergency because the number of cases in other countries was small, and there was as yet no person-to-person spread outside of China.
So far, 99% of confirmed cases are still in China. Officials had been following contacts of the five cases in anticipation that the virus might have spread before the original patient was confirmed to have the coronavirus and isolated.
Tedros noted he felt encouraged by President Xi's involvement in the response to the virus.
Dr. Tedros stressed that the declaration is, "not a vote of no confidence in China", a country he praised for its swift and transparent response to the outbreak.