(CNN) An Ebola outbreak in the Democratic Republic of Congo has entered a "new phase" after it spread to a large city, the country's health minister, Dr. Oly Ilunga Kalenga, said Wednesday.

The new case of Ebola virus disease has been confirmed in Wangata, one of the three health zones of Mbandaka, a city of nearly 1.2 million people in Equateur Province in northwestern Congo, the World Health Organization confirmed Thursday.

The spread of the virus from rural areas into a city has raised fears it could quickly spread and become harder to control.

A total of 45 cases of hemorrhagic fever have now been reported, including 25 deaths, the health ministry said Thursday. Fourteen cases have been confirmed with laboratory tests.

Until now, the cases and deaths were reported from the rural Bikoro health zone, nearly 150 kilometers from Mbandaka, allowing authorities to attempt ring-fencing vaccinations in the the affected areas.

A newly confirmed case in a densely populated part of the country will complicate attempts to control the outbreak.

"This is a concerning development, but we now have better tools than ever before to combat Ebola," said Dr. Tedros Adhanom Ghebreyesus, the WHO's director-general. "WHO and our partners are taking decisive action to stop further spread of the virus."

The WHO is deploying about 30 experts to conduct surveillance in the city and inform local communities on treatment and prevention methods in collaboration with the country's Ministry of Health.

The UN agency is also partnering with nongovernmental organizations, including Médecins Sans Frontières, to ensure that health facilities are ready to treat patients in isolation wards.

"The arrival of Ebola in an urban area is very concerning and WHO and partners are working together to rapidly scale up the search for all contacts of the confirmed case in the Mbandaka area," said Dr. Matshidiso Moeti, the WHO's regional director for Africa.

"With the new case confirmed in Mbandaka, the scenario has changed," said Henry Gray, an MSF emergency coordinator in Mbandaka. "It is paramount to trace the suspect case in order to have a clearer view on how it reached the city."

In Mbandaka and Bikoro, 514 people who may have been in contact with infected people have been notified by national health authorities and are being monitored, according to MSF.

Isolation zones have been set up in Mbandaka's main hospital as well as Bikoro hospital, with more Ebola treatment centers being set up in Mbandaka and Bikoro.

The Department of Health first reported the outbreak, the ninth the country has faced in the past four decades, on May 8.

"This outbreak is very close to the Republic of Congo and Central African Republic. And we are taking it very seriously," Dr. Ibrahima-Soce Fall, the WHO regional emergency director for Africa, said Monday when two cases were suspected in Mbandaka and being kept in isolation.

The Ebola vaccine being provided -- called rVSV-ZEBOV -- has been shown to be safe in humans and highly effective against the Ebola virus, according to the WHO.

A 2016 study found it to be 100% effective in trials in Guinea in coordination with the country's Ministry of Health after the 2014-15 outbreak.

The UN agency has been working with the Ministry of Health and Médecins Sans Frontières to conduct the ring fence vaccinations across the affected areas -- where contacts of those infected, followed by contacts of those contacts, would all be vaccinated.

More doses of vaccine are expected to be shipped out, a WHO spokesman said earlier in the week.

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Ebola virus disease, which most commonly affects people and nonhuman primates such as monkeys, gorillas and chimpanzees, is caused by one of five Ebola viruses. On average, about 50% of people who become ill with Ebola die.

The first human outbreaks of Ebola occurred in 1976, one in the north of what is now Congo and in the region that is now South Sudan.

Humans can be infected by other humans if they come in contact with body fluids from an infected person or contaminated objects from infected persons. Humans can also be exposed to the virus, for example, by butchering infected animals.

West Africa experienced the largest recorded outbreak of Ebola over a two-year period beginning in March 2014; a total of 28,616 confirmed, probable and suspected cases were reported in Guinea, Liberia and Sierra Leone, with 11,310 deaths, according to the WHO.