World Health Organization (WHO) Director-General Tedros Adhanom Ghebreyesus gestures during a daily press briefing on COVID-19 virus at the WHO headquaters on March 2, 2020 in Geneva.

"This epidemic can be pushed back but only with a coordinated and comprehensive approach that engages the entire machinery of government," WHO Director-General Tedros Adhanom Ghebreyesus said during a press briefing at the agency's headquarters in Geneva. "We're calling on every country to act with speed, scale and clear-minded determination."

The World Health Organization on Thursday called on all nations to "pull out all the stops" to fight the COVID-19 coronavirus as it continues to spread outside of China.

Tedros said world health officials are "deeply concerned" about the increasing number of countries reporting cases, especially those with weaker health-care systems. He's also worried that some countries aren't taking this seriously enough or have decided that there's nothing they can do to curb local outbreaks.

"This epidemic is a threat for every country, rich and poor. And as we've said before, even the high-income countries should expect surprises," he said. "We're concerned that in some countries, the level of political commitment and the actions that demonstrate that commitment do not match the level of the threat we all face."

"This is not a drill. This is not the time to give up. This is not a time for excuses. This is a time to pull out all the stops," he said.

WHO officials last week increased the risk assessment of the coronavirus to "high" to "very high" at a global level. The world can still avoid "the worst of it," but the increased risk assessment means the WHO's "level of concern is at its highest," Dr. Mike Ryan, executive director of WHO's health emergencies program, said at the time.

Tedros said Thursday that the number of newly reported cases in South Korea appears to be declining, adding the new cases identified are primarily from known clusters.

This virus "is not a one-way street," he said. "This virus can be pushed back."

World health officials said Tuesday the mortality rate for COVID-19 is 3.4% globally, higher than previous estimates of about 2%.

During a press briefing Monday, WHO officials said they don't know how COVID-19 behaves, saying it's not like influenza. They added that while much is known about the seasonal flu, such as how it's transmitted and what treatments work to suppress the disease, that same information is still in question when it comes to the coronavirus.

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. Maria Van Kerkhove, head of WHO's emerging diseases and zoonosis unit, said Thursday scientists currently do not believe that asymptomatic transmission is a "major driver" of the disease, adding, "that could change."

"It's still early days, it's still a few weeks into this outbreak," she said. "We're learning something new every day about this virus."