KINSHASA, Congo — Congo’s Ebola outbreak has spread to a crossroads city of more than 1 million people in a troubling turn that marks the first time the vast, impoverished country has encountered the lethal virus in an urban area.

“This is a major, major game-changer in the outbreak,” Dr. Peter Salama, the World Health Organization’s deputy director-general of emergency preparedness and response, warned on Thursday.

A single case of Ebola was confirmed in Mbandaka, a densely populated provincial capital on the Congo River, Congo’s Health Minister Oly Ilunga said late Wednesday. The city is about 150 kilometers (93 miles) from Bikoro, the rural area where the outbreak was announced last week.

Medical teams rushed to track down anyone thought to have had contact with infected people, while WHO continued shipping thousands of doses of an experimental vaccine.

A total of 44 cases of Ebola have been reported in Congo in this outbreak: three confirmed, 20 probable and 21 suspected, according to WHO. Twenty-three of those people have died.

Until now, the outbreak was confined to remote rural areas, where Ebola, which is spread by bodily fluids, travels more slowly.

“We’re certainly not trying to cause any panic in the national or international community,” Salama said. But “urban Ebola can result in an exponential increase in cases in a way that rural Ebola struggles to do.”

Mbandaka, a city of almost 1.2 million, is in a busy travel corridor in Congo’s northwest Equateur province and is upstream from the capital, Kinshasa, a city of about 10 million. It is an hour’s plane ride from Kinshasa or a four- to seven-day trip by river barge.

Salama also noted Mbandaka’s proximity to neighboring countries, including Central African Republic and Republic of Congo.

“The scenario has changed, and it has become most serious and worrying, since the disease is now affecting an urban area,” said Henry Gray, emergency coordinator in Mbandaka for Doctors Without Borders.

The aid organization said 514 people believed to have been in contact with infected people are being monitored. WHO said it is deploying about 30 more experts to the city.

Those exposed will for the first time in Congo receive Ebola vaccinations, the health minister said. WHO has sent 4,000 doses to Congo and said it will dispatch thousands more in the coming days as needed.

“This is a concerning development, but we now have better tools than ever before to combat Ebola,” Tedros Adhanom Ghebreyesus, WHO director-general, said of the new urban case.

The vaccine has been shown to be highly effective against Ebola. It was tested in Guinea during the outbreak that killed more than 11,300 people in West Africa from 2014 to 2016.

WHO has said it will use the “ring vaccination” method. It involves vaccinating contacts of those feared infected, contacts of those contacts, and health care and other front-line workers.

This is the ninth Ebola outbreak in Congo since 1976, when the disease was first identified. The virus is initially transmitted to people from wild animals, including bats and monkeys.

There is no specific treatment for Ebola. Symptoms include fever, vomiting, diarrhea, muscle pain and at times internal and external bleeding. The virus can be fatal in up to 90 percent of cases, depending on the strain.

Petesch reported from Dakar, Senegal. AP reporter Jamey Keaten in Geneva contributed to this report.

READ MORE: Democratic Republic of Congo reports Ebola outbreak, 2 cases confirmed