More than 100,000 coronavirus cases were reported to the World Health Organization in the previous 24 hours, "the most in a single day since the outbreak began," Director-General Tedros Adhanom Ghebreyesus said Wednesday.
"We still have a long way to go in this pandemic," he said at a news conference in Geneva. "In the last 24 hours, there have been 106,000 cases reported to WHO — the most in a single day since the outbreak began."
Tedros added that almost "two-thirds of these cases were reported in just four countries," although he did not specify where the cases had been recorded.
The countries with the highest numbers of confirmed cases are the U.S., Russia, Brazil and the U.K., according to the WHO.
"We are very concerned about rising cases in low- and middle-income countries," Tedros said.
"The pandemic has taught and informed many lessons," he added. "Health is not a cost. It's an investment. To live in a secure world, guaranteeing quality health for all is not just the right choice. It's the smart choice."
At the briefing, Dr. Mike Ryan, head of WHO's emergencies program, said the "tragic milestone of 5 million cases" would soon be hit.
He added that people should avoid using the malaria medicine hydroxychloroquine, except for conditions it is approved to treat, such as lupus and rheumatoid arthritis. President Donald Trump said Monday that he was taking hydroxychloroquine to prevent coronavirus infection.
Tedros acknowledged that he had received a letter from Trump, who threatened this week to withdraw the U.S. from the WHO and permanently withhold funding. Trump has accused the organization of mishandling the outbreak and favoring China.
Tedros declined to comment further on the letter, but he said the WHO was "looking into it."
The WHO has announced a review of its response to the pandemic, which emerged in China late last year.
"I said it time and time again that WHO calls for accountability more than anyone. It has to be done, and when it's done it has to be a comprehensive one," Tedros said of the review, while declining to give a timeline.
Download the NBC News app for full coverage and alerts about the coronavirus outbreak
In the U.S., the Centers for Disease Control and Prevention quietly released more reopening guidance with advice for child care facilities, schools, day camps, mass transit systems, restaurants, bars and other businesses and organizations whose workers are at high risk of becoming sick from the coronavirus.
The guidance offers safety measures to protect employees and other people as states relax stay-at-home orders aimed at stopping the spread of the virus.