The Democratic Republic of Congo warned late on Wednesday that the nation's Ebola outbreak had entered a "new phase" after a case was discovered in the northwestern city of Mbandaka.

It was the first registered urban case of the deadly virus, as the city has a population of some 1 million people.

"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 was quoted by the Reuters news agency as saying.

Kalenga added that the authorities were tracing all air, river, and road routes in and out of the city in order to trace the source of the virus.

At least 23 people have died in Congo's latest outbreak, which had originally been detected in only rural areas — where it would have given health officials a better chance of containing the spread of Ebola.

Experimental vaccines arrive

Also on Wednesday, the World Health Organization (WHO) announced that it was intensifying its vaccination campaign in Congo. A first batch of 4,000 inoculations has arrived in the capital Kinshasa, with another 4,000 to follow soon after.

The experimental vaccines were developed two years ago following the deaths of 11,300 people in Guinea, Liberia, and Sierra Leone in a 2014-2016 outbreak that was the deadliest in the virus' history.

Ebola virus disease spreads between humans and other infected animals via bodily fluids and can cause severe fevers, vomiting, and internal bleeding. It was named for the Ebola River in Congo, where it was first identified in 1976.

es/rc (Reuters, AP)