The Democratic Republic of Congo's Ebola outbreak has spread from the countryside to Mbandaka, a city of a million people approximately 130 km from where earlier cases this month were confirmed, Health Minister Oly IIunga Kalenga has said.

Forty-two people are now infected and 23 people have already died from the disease, and fears have spiked that it could spread further as Mbandaka is a major transportation hub on the way to the capital Kinshasa.

Senior World Health Organisation official Peter Salama warned that the spread to Mbandaka meant there was the potential for an "explosive increase" in cases.

"This is a major development in the outbreak," he told the BBC. "We have urban Ebola, which is a very different animal from rural Ebola. The potential for an explosive increase in cases is now there."

Ebola, a serious infectious illness that causes internal bleeding and often proves fatal, can spread rapidly through contact with small amounts of bodily fluid and its early flu-like symptoms are not always obvious.

Approximately 11 300 people were killed between 2014 and 2016 in the West African outbreak of the disease as it spread to the capital cities of Guinea, Sierra Leone and Liberia.