The head of the World Health Organization said on Wednesday that there was little risk of the Ebola outbreak in central Africa developing into a global pandemic, even as the number of suspected cases and deaths continued to rise.

The outbreak is “not a pandemic emergency,” Tedros Adhanom Ghebreyesus, the W.H.O.’s director general, said at a news conference in Geneva. He said the agency had assessed the risk as “high at the national and regional levels, and low at the global level.”

By Wednesday, five days after the outbreak was declared, the number of suspected infections had risen to nearly 600, including 139 deaths, Dr. Tedros said. He said the outbreak was concentrated in two provinces of the Democratic Republic of Congo, Ituri and North Kivu, including their respective capitals. Neighboring Uganda has also reported two cases in travelers from Congo, one of whom has died, he said.

“We expect those numbers to keep increasing, given the amount of time the virus was circulating before the outbreak was detected,” Dr. Tedros said.

Global health officials have said that the outbreak could last for months, and that many infections may have gone unreported.

Dr. Tedros also defended the agency’s response to the outbreak, after Agence France-Presse quoted Marco Rubio, the U.S. secretary of state, as saying on Tuesday that the W.H.O. had been late to identify it. The New York Times has not confirmed that Mr. Rubio made the remarks as quoted.

Asked about the reported comments, Dr. Tedros said they may have reflected a “lack of understanding” about how the W.H.O. functions. “We don’t replace the country’s work, we only support them,” he said, referring to the health authorities in countries where an outbreak begins.

The United States formally withdrew from the W.H.O. in January. That deprived it of its biggest funding source and saw it scramble to cut its 2026-27 budget by $500 million. The Trump administration also shut down the U.S. Agency for International Development last year, an organization that has played a major role in containing previous outbreaks. The United States also cut funding for the U.S. Centers for Disease Control and Prevention.

Dr. Peter Chin-Hong, an infectious diseases specialist at the University of California, San Francisco, said those actions “have resulted in the fundamental weakening of the W.H.O. for surveillance, staffing, logistics, laboratory support and rapid response coordination across multiple countries.”

Asked on Wednesday whether the public health response had been hampered by U.S. funding cuts, Dr. Tedros said that given the complexity of the circumstances surrounding the outbreak, “it would be very difficult to associate it with funding alone.”

Ituri, a province in Congo’s northeast, where the outbreak is centered, is a challenging place to monitor and control the spread of an outbreak. Large numbers of people have been displaced by years of conflict. The region’s gold mines draw migrant laborers from elsewhere.

Local officials have also said that equipment in the province could only test for the most common species of Ebola, known as Zaire, not the relatively rare species, known as Bundibugyo, which is responsible for the current outbreak. Because of that, early results returned negative readings. Diagnostic kits capable of identifying Bundibugyo were flown in over the weekend, a W.H.O. representative said.

Dr. Tedros said that while health officials in Congo had a track record of quickly identifying and responding to Ebola outbreaks, the conditions around this one had made the situation “more complex.”

Because of the added risk factors, Dr. Tedros said, he took the unprecedented step on Sunday of declaring the outbreak a “public health emergency of international concern” before a W.H.O. emergency committee had met to discuss the matter.

On Tuesday, the U.S. State Department announced that it would fund up to 50 treatment clinics and cover associated frontline costs in Congo and Uganda, to provide emergency Ebola screening, triage and isolation capacity.

An American missionary doctor in Ituri Province, Peter Stafford, was infected while treating patients there, officials said. He was flown for treatment to Germany. A spokesman for Charité hospital in Berlin confirmed on Wednesday that an American Ebola patient had arrived overnight, without giving his name. The hospital has extensive experience treating patients with highly infectious diseases, including Ebola.

Nick Cumming-Bruce and Christopher F. Schuetze contributed reporting.