Public health officials are sounding new warnings about the spread of the deadly Ebola virus in an eastern province of Congo as cases pop up in a city of more than a million people and as stockpiles of a lifesaving vaccine dwindle.

Congolese Health Minister Oly Ilunga Kalenga said at a news conference Thursday that health workers had seen a dramatic spike in cases in Butembo, a regional trading hub with strong links to neighboring Uganda and Rwanda.

The Health Ministry said 25 confirmed cases of the virus have been identified in Butembo, up from 11 cases two weeks ago.

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Kalenga said the community in Butembo has been reluctant to acknowledge the virus’s presence, and interactions with health-care workers either trying to treat patients or vaccinate those who have come into contact with patients have turned violent. He said several health workers have been attacked, and health centers have been destroyed.

Doctors Without Borders, which is operating several Ebola treatment units in the heart of the epidemic in North Kivu, said the number of cases has risen especially in an eastern suburb of Butembo.

“We are very concerned by the epidemiological situation in the Butembo area,” said John Johnson, the lead Doctors Without Borders official in the city. “We’re expecting this outbreak will last for a while, and we must increase our efforts to get it under control.”

In the more than four months since the virus broke out in North Kivu and neighboring Ituri provinces, 471 people have been infected and 273 have died.

The current outbreak has become the worst in the history of Congo, and the second-worst Ebola epidemic in modern history, after an outbreak in West Africa killed more than 11,300 people several years ago.

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An experimental vaccine developed by the U.S. government and manufactured by the pharmaceutical company Merck has likely saved thousands of lives, Kalenga said Thursday. He said that without the vaccine, which has already been administered to more than 40,000 people around the affected provinces, as many as 10,000 people might have been infected.

But there are troubling signs that the stockpile of that vaccine is diminishing, as Merck races to produce more. The company has said it is committed to maintaining 300,000 doses to fight future Ebola outbreaks.

But many more may be needed if the virus breaks out in a city as large as Butembo — or if it spreads across international borders into Uganda, Rwanda, Burundi or South Sudan.

“We are extremely concerned about the size of the vaccine stockpile,” Peter Salama, the World Health Organization’s (WHO) deputy director-general of emergency preparedness and response, told STAT News in an interview this week.

Kalenga said health-care workers are already on the ground in Goma, the capital of North Kivu Province and another city of more than a million residents. Goma sits on Lake Kivu, on the Congolese border with Rwanda.

American aid agencies like the Centers for Disease Control and Prevention (CDC) and the U.S. Agency for International Development have largely stayed on the sidelines during the outbreak, because of unrest in a province long riven by sectarian and ethnic tensions. Several American aid workers were pulled back to their base in Kinshasa in August after a rebel attack on a Congolese military base.

In September, a terror attack in Beni, at the heart of the outbreak, killed almost two dozen people. In October, several Congolese health-care workers were killed in an ambush. Several international health-care workers told The Hill they routinely hear gunfire in the streets of Beni.

“The security situation is indeed tense and it has been there for many, many years,” Salama told The Hill last month. “The population is extremely tense.”

North Kivu Province is the largest province in Congo outside of the capital of Kinshasa. More than a million of the 8 million residents are internally displaced by ethnic violence, according to the United Nations Office for the Coordination of Humanitarian Affairs.

The U.S. government is funding and running a CDC-led vaccination campaign for medical personnel in Uganda, in case the virus does jump international borders.

The White House said this month that about three dozen American officials, mostly from the CDC, are working to support the outbreak response from Kinshasa and Washington, D.C., and the CDC has dispatched other workers to WHO’s headquarters in Geneva.

Kalenga said he expected the outbreak to last for several months. The risk of further spread, he said, will remain high until the virus is contained.