LONDON— The emergencies chief of the World Health Organization said Central and South America are currently witnessing the most intense transmission of the coronavirus worldwide, but it’s difficult to predict when the epidemic might peak there.

In the last 24 hours, Dr. Michael Ryan said five of the 10 countries reporting the highest number of cases are in the Americas: the U.S., Brazil, Peru, Chile and Mexico. He said that while the growth of COVID-19 was not exponential in all those countries, officials were seeing a progressive increase in cases and that hospitals were starting to strain under the pressure.

“We’re particularly concerned about places like Haiti because of the inherent weaknesses in the system,” Ryan said at a press briefing on Monday. “I think we now absolutely need to focus on supporting particularly Central and South America,” he said. He added that while officials previously had very serious concerns about COVID-19’s impact in South Asia and Africa, outbreaks in those regions, although difficult, were now stable.

“I don’t believe we’ve reached the peak” in the Americas, Ryan said, noting that several factors in the region, including the number of urban poor and fragile health systems, made outbreaks in those countries particularly dangerous.