KINSHASA, May 17 (Xinhua) -- The Ebola outbreak of the Democratic Republic of the Congo (DRC) has entered a "new phase" following the first reported case of the deadly virus in the northwestern city of Mbandaka, with a population of some 1 million, the health minister said Wednesday.

"We are entering a new phase of the Ebola outbreak that is now affecting three health zones, including an urban health zone," Health Minister Oly Ilunga Kalenga said in a statement.

The 23 people, who have died in the DRC's latest outbreak, had been detected in rural areas, giving authorities a better chance of containing the spread of the virus.

The first urban case threatens to change that. Mbandaka is a major transportation hub with routes to the capital Kinshasa. Located on the banks of the Congo River, the Republic of Congo is on the other side of the river.

"This is a major development in the outbreak, We have urban Ebola, which is a very different animal from rural Ebola. The potential for an explosive increase in cases is now there," BBC quoted Senior World Health Organization (WHO) official Peter Salama as saying.

Kalenga said authorities were tracing all air, river and road routes in and out of the city to find the source of the virus.

"Since the announcement of the alert in Mbandaka, our epidemiologists are working in the field to identify people who have been in contact with suspected cases," Kalenga said.

The DRC is now in its ninth Ebola outbreak since 1976.

The first 4,000 doses of experimental vaccine from the WHO to combat the Ebola outbreak has arrived Wednesday in Kinshasa. Salama said another 4000-vaccine batch will arrive soon.

The vaccine, developed by Merck and Co Inc, is still not licensed but has proved effective during limited trials in West Africa in the biggest-ever outbreak of Ebola, which killed 11,300 people in Guinea, Liberia and Sierra Leone from 2014 to 2016.

Ebola is a highly infectious disease spread through contact with even small amounts of bodily fluid of an infected person. Its early flu-like symptoms are not always easy to detect.