"We've already seen in countries, and quite sophisticated countries, who've had a rapid rise in cases in the last week are having trouble coping with the clinical case loads," he said during a press briefing at the agency's headquarters in Geneva. "We need to keep this virus slowed down, because health systems around the world, and I mean North and South, are just not ready."

Health systems, even in more advanced countries, are "just not ready" for a COVID-19 epidemic, said Dr. Mike Ryan, executive director of the World Health Organization's health emergencies program.

Hospitals across the world are not prepared to handle the coronavirus outbreak that is migrating from Asia to continents across the world, World Health Organization officials said Friday.

The remarks follow the rapid spread of the virus beyond China. On Friday morning, WHO director-general Tedros Adhanom Ghebreyesus said there are 4,351 cases and 67 deaths in 48 countries outside of China. That's up from 505 cases across 24 countries just two weeks ago.

Since Thursday, Denmark, Estonia, Lithuania, Netherlands and Nigeria all reported their first cases of the virus. Just before the WHO's Friday news briefing, Mexico's Health Ministry confirmed the nation's first two cases of the virus, making it the second country in South America to report infections.

"Our determination right now is that health systems around the world are not ready and need to be better prepared to absorb the impact of the virus," Ryan added. "People need to take a reality check now."

"What we see at the moment are linked epidemics of COVID-19 in several countries," Tedros said Friday, "but most cases can still be traced to known contacts or clusters of cases."

Rapid spread of the virus in Italy, Iran and South Korea are all cause for concern, Tedros said on Thursday.

"We're at a decisive point," he said. "The epidemics in the Islamic Republic of Iran, Italy and the Republic of Korea demonstrate what this virus is capable of."

WHO officials on Friday also increased the risk assessment of the coronavirus to "very high" at a global level. It is the highest level risk assessment the WHO can assign to the outbreak.

"We are on the highest level of alert or highest level of risk assessment in terms of spread and in terms of impact," Ryan said. That said, the WHO isn't trying to alarm or scare people, he said. "This is a reality check for every government on the planet: Wake up! Get ready! This virus may be on its way, and you need to be ready. You have a duty to your citizens, you have a duty to the world to be ready."

Ryan added that data do not currently support health officials declaring the virus a global pandemic.

"If we say there's a pandemic of coronavirus, we're essentially accepting that every human on the planet will be exposed to that virus," he said. "The data does not support that as yet and China have clearly shown that that's not necessarily the natural outcome of this event if we take action, if we move quickly, if we do the things we need to do."

— CNBC's Berkeley Lovelace and Noah Higgins-Dunn contributed to this article.