World Health Organization (WHO) Director-General Tedros Adhanom Ghebreyesus talks during a daily press briefing on COVID-19 virus at the WHO headquaters in Geneva on March 11, 2020.

Calling the coronavirus pandemic "Public Enemy No. 1," World Health Organization officials warned Wednesday against squandering a narrowing window of opportunity for countries and their citizens to limit the final death toll of the outbreak by taking aggressive actions to slow the virus's spread.

"It's a dangerous virus. We had been saying to the world the window of opportunity is narrowing and the time to act was actually more than a month ago, two months ago," WHO Director-General Tedros Adhanom Ghebreyesus said at a news briefing.

Tedros noted, "The pandemic continues to take a massive toll."

"We have overcome many pandemics and crises before. We will overcome this one, too. The question is how large a price we will pay," he said.

"We have lost more than 16,000 lives, we will lose more. How many more will be determined by the decisions we make," Tedros said.

In fact, the global death toll from COVID-19 is now nearly 20,000, according to Johns Hopkins University data.

Global infections from the virus have risen to more than 441,000, according to JHU.

Dr. Mike Ryan, executive director of WHO's health emergencies program, told reporters that, "Unfortunately the world was not ready for a pandemic."

"Not being ready we don't have the security stockpiles in place that are immediately deployable in order to scale up our capacity to protect our front-line health workers and others," Ryan said. "There are shortages of PPE (personal protective equipment), shortages of ventilators and other products for the medical response to COVID."

Tedros said it was crucial that political leaders of nations take actions to slow the spread of the pandemic to avoid overwhelming their countries' health-care systems.

Countries around the world have shuttered schools, closed nonessential businesses and restricted travel. But the extent of those restrictions has varied widely, including within individual countries.

Asked about actions taken in the United States, Ryan said, "We remain impressed by the work that's being done at the state and local level."

Tedros lamented the delay in getting enough national governments to respond aggressively to the outbreak after it began several months ago in China.

"We have been saying for more than two months now this virus is 'Public Enemy No. 1,'" Tedros said.

"In some countries the number of cases have really jumped and overwhelmed the system and they were not prepared so it was really difficult to give service to the patients who were coming to the hospitals to get service," Tedros said.

"So preparing the system is very important for any country."

He later said, "I think we squandered the first window of opportunity but we are saying today ... this is a second opportunity that we should not squander."

WHO officials reiterated that every country should do its best to test as many people as it can so it can isolate positive cases and prevent further spread.

Ramping up the production and availability of testing remains one of the key actions the WHO recommends.

"It's critical that you test to find where this virus is so you know where you're fighting it," Dr. Maria Van Kerkhove, head of the WHO's emerging diseases and zoonosis unit, said.

On Monday, Tedros had noted that, "It took 67 days from the first reported case to reach 100,000 cases, 11 days for second 100,000 cases, and just four days for the third 100,000 cases."

On Wednesday, Tedros said that the International Olympic Committee's decision to postpone the Olympics was "difficult but wise," noting that it will protect athletes, spectators and officials.