A San Francisco Public Works Community Clean volunteer wearing a face mask removes trash on a street in the Chinatown district of San Francisco, California, on Wednesday, Feb. 26, 2020.

Tedros said the biggest concerns now are what's happening outside China, where the growth in cases have slowed. Brazil, Georgia, Greece, North Macedonia, Norway, Pakistan and Romania reported their first coronavirus cases in the last day, Tedros said.

"It does not distinguish between races or ethnicities. It has no regard for a country's GDP or level of development," he added.

"Every country must be ready for its first case," Tedros Adhanom Ghebreyesus, director-general of WHO, said during a press briefing at the agency's headquarters in Geneva. "No country should assume it won't get cases. That could be a fatal mistake. This virus does not respect borders."

World Health Organization officials warned on Thursday that member nations need to prepare for COVID-19's arrival after seven countries in the last day reported their first cases.

"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."

On Wednesday, WHO officials said the number of new COVID-19 cases outside China exceeded those inside the country for the first time. "The sudden increases of cases in Italy, the Islamic Republic of Iran and the Republic of Korea are deeply concerning," Tedros said at the time.

That same day, U.S. health officials confirmed the first possible community transmission of the coronavirus in America, a troubling sign that the virus could be spreading in local cities and towns. The California patient didn't have a relevant travel history or exposure to another patient with the virus, the CDC said.

Tedros said Thursday that countries must act "swiftly" and "aggressively" to contain the virus.

"With the right measures, it can be contained," he said, adding countries should begin thinking about whether they have proper isolation units, medical supplies and other vital equipment.

The preparation will "be the difference between one case and 100 cases in the coming days and weeks," he said.

Health officials have said the respiratory disease is capable of spreading through human-to-human contact, droplets carried through sneezing and coughing and germs left on inanimate objects.

Dr. Mike Ryan, executive director of the World Health Organization's emergencies program, said some countries have experienced localized clusters.

"Then there are other countries like [South] Korea and Japan where there is mainly cluster of disease but there's some evidence of transmission at the community level."

Tedros reiterated that the virus has "pandemic potential," adding, "this is not a time for fear. This is the time for taking action now to prevent infections and save lives now."

"Fear and panic doesn't help," he said. "People can have concerns and rightly so. People can be worried and rightly so. But the most important thing is to calm down and do the right things to fight this very dangerous virus."