The risk of the coronavirus outbreak's global spread and impact is at its highest possible level, the World Health Organisation (WHO) says.
The level has been raised to "very high" from "high" by WHO and brings the rest of the world in line with China, which was already at "very high".
This means that an "immediate response" - within hours - is required as soon as a COVID-19 case is suspected.
However, it would be a "big mistake" to switch from a public health strategy of containment to mitigation, where authorities accept the coronavirus is spreading, WHO director general Dr Tedros Adhanom Ghebreyesus said.
He added: "Our epidemiologists have been monitoring these developments continuously, and we have now increased our assessment of the risk of spread and the risk of impact of COVID-19 to very high at a global level.
"We are not underestimating the risk, that is why we said today the global risk is very high; we increased it from high to very high."
Dr Mike Ryan, WHO's emergency programme chief, added: "We are on the highest level of alert and highest level in terms of spread and of impact. But that is not in order to alarm or scare people.
"People need to take a reality check now and really understand an all-of-government, an all-of-society approach is needed."
More than 83,800 cases have been confirmed and nearly 2,900 deaths recorded around the world since the virus was first detected in Wuhan, central China, in December.
While most cases have been in China, hotspots for the virus also now include Italy, Iran and South Korea.
On Friday, four more people died from COVID-19 in Italy's Lombardy region, bringing the country's total number of dead from the coronavirus to 21.
Dr Ryan said Iran's outbreak may be worse than realised, with 34 dead, making it the highest number of deaths outside China.
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A WHO mission to Iran has been delayed due to travel restrictions but they are hoping to get there on Sunday at the earliest, with help from the United Arab Emirates.
South Korea has reported the highest number of cases outside China, with 571 new infections on Friday, bringing the total to 2,337, with 13 deaths.
China reported 329 cases in the past 24 hours, the lowest in the country in more than a month.
Nigeria reported its first case on Friday - linked to Italy - but Dr Ryan said it had "well-tested mechanisms for dealing with these dangerous pathogens", citing experience with Lassa fever and cholera.
Mr Adhanom Ghebreyesus said most confirmed cases of COVID-19, at the moment, are linked to "known contacts or clusters of cases".
"We do not see evidence as yet that the virus is spreading freely in communities," he added.
"As long as that's the case, we still have a chance of containing this coronavirus, if robust action is taken to detect cases early, isolate and care for patients and trace contacts.
"The key to containing this coronavirus is to break the chains of transmission."
He also announced that more than 20 vaccines are being developed around the world, with several treatments in clinical trials, and results are expected in a few weeks.
But, he said people can protect themselves ahead of those results by regularly washing your hands, disinfecting surfaces regularly, not touching your face, watching out for symptoms, avoiding travel if you have a fever or cough, sneezing into a tissue and avoiding crowded places if you are over 60 years old or have underlying conditions.