Health officials in the Democratic Republic of the Congo (DRC) said Thursday that an outbreak of the Ebola virus had spread to an important port city, opening a new and potentially deadly front in an epidemic that had so far been contained within small communities.

The new case is in Mbandaka, a city of about 1.2 million residents. The virus's presence is dangerous there because Mbandaka is a port on the Congo River, about 300 miles upstream from the capital, Kinshasa. Mbandaka also has a commercial airport served by two different airlines.

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Tedros Adhanom Ghebreyesus, the new director general of the World Health Organization, called the spread of the virus to such a large city "a concerning development."

In the eight previous Ebola outbreaks in the DRC over the last 40 years, the virus has never reached a major city.

"With the new case confirmed in Mbandaka, the scenario has changed, and it has become more serious and worrying, since the disease is now affecting an urban area. It is paramount to trace the suspect case in order to have a clearer view on how it reached the city," said Henry Gray, emergency coordinator in Mbandaka for Doctors Without Borders.

At least 44 people have shown symptoms of a viral hemorrhagic fever since the beginning of April, according to the latest tally. The first cases emerged in a village near the town of Bikoro, about 50 miles southwest of Mbandaka.

Twenty-three people have died during the outbreak. Three of those cases have tested positive for the Ebola virus; the rest are likely to have Ebola as well, though laboratories have yet to confirm the virus's presence.

The DRC's Health Ministry, the World Health Organization, Doctors Without Borders and the Centers for Disease Control and Prevention (CDC) have been tracking the virus since an initial investigative team arrived in Bikoro on May 5. At the time, that team found five cases in Bikoro and the smaller village, Ikoko Impenge.

Doctors Without Borders said the groups were monitoring 514 people who had come into contact with those who were infected, in hopes of containing the virus before it spread more widely.

Doctors Without Borders has set aside five beds for treating potential Ebola patients in a hospital in Mbandaka, and another 10 beds in Bikoro. The group is building two Ebola treatment centers capable of handling 20 patients, one each in Mbandaka and Bikoro.

The WHO has deployed 50 public health experts to the three cities. The agency is also operating daily flights to race supplies and support staff from Kinshasa. The WHO, the United Nations and the Wellcome Trust, a British-based nongovernmental organization, have collectively released about $5 million to fight the outbreak, said Tarik Jasarevic, a WHO spokesman.

Jasarevic said an initial batch of 4,000 doses of a new Ebola vaccine is already on the way to the affected region. He said it would be a challenge to land vaccination teams in such a remote area, but that the WHO and groups like Gavi, a public-private partnership that delivers vaccines, were working through the logistics.

The outbreak is the third in the DRC in the last four years.

Tom Frieden, the former head of the CDC who now runs the public health group Resolve to Save Lives, said the response in the last few weeks is better than the response to the outbreak in West Africa four years ago, which killed at least 11,300 people — and likely tens of thousands more — in Liberia, Guinea and Sierra Leone.

But he said the fact that it took so long for health officials to identify the virus illustrated a weakness in the global public health system.

"We are doing better at response, but not much better at rapid detection, which is important," Frieden said in an email. "This was spreading for a while before [it was] recognized."

One of the driving factors that allowed the outbreak in West Africa to spread so widely between the three countries was that Ebola was not thought to be endemic to West Africa. Health officials did not know they were looking at Ebola cases in the early months of 2014, when the virus spread from a tiny hillock in rural Guinea across borders into Liberia and Sierra Leone.

But Ebola is endemic to the Congo River basin; the first identified outbreak in modern times took place about 300 miles from the current site, back in 1976.