The WHO chief also announced a new official name for the disease, saying that the agency has dubbed it COVID-19.
WHO director-general Tedros Adhanom Ghebreyesus saying the epidemic was "very much an emergency" for China but also "one that holds a very grave threat for the rest of the world".
The WHO had advised several guidelines for the new name, warning that it should not include geographical locations, people's names, the name of an animal or a kind of food, or references to a particular culture or industry.
The WHO had earlier given the virus the temporary name of "2019-nCoV acute respiratory disease" and China's National Health Commission this week said it was temporarily calling it "novel coronavirus pneumonia" or NCP.
The announcement came as the death toll in mainland China has now reached more than 1,000, after 108 people died from the virus on Monday - the highest daily toll since the outbreak began late previous year in the central Chinese city of Wuhan.
"If we invest now, we have a realistic chance of stopping this outbreak", Ghebreyesus said.
"This is of course too many", the World Health Organization chief said of the fatalities, noting that many questions still needed answering, such as where the outbreak was growing and where it was getting better or worse.
"As of 6am Geneva time this morning, there were 42,708 confirmed cases reported in China, and tragically we have now surpassed 1000 deaths - 1017 people in China have lost their lives to this outbreak".
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"This may seem like a trivial issue to some, but disease names really do matter to the people who are directly affected", WHO's assistant director-general for health security Keiji Fukuda said at the time.
The virus has already paralyzed China's economy as many large companies urged their employees to stay at home in order to curb the spread of the disease.
Director General Tedros Adhanom Ghebreyesus traveled to Beijing for talks with President Xi Jinping and Chinese ministers in late January and returned with an agreement to send an global mission.
Experts say it could still be months or even years before any approved treatments or vaccines are developed.
World Health Organization chief Tedros Adhanom Ghebreyesus was less sanguine, however, appealing for the sharing of virus samples and speeding up of research into drugs and vaccines.
Tedros met with Chinese President Xi Jinping in Beijing last month and said it took about two weeks for the country to agree to allow an worldwide team of experts to come to investigate the outbreak.
Chinese authorities, meanwhile, dismissed two senior health officials from Hubei, the central province where some 56 million people, including in its capital Wuhan, have been under lockdown since late last month. Just under 400 of those cases have been confirmed in countries outside of China.