The Ebola outbreak in Congo has spread from the countryside into a city, prompting fears that the disease will be increasingly difficult to control.
The health minister Olly Ilungu Kalenga confirmed a case in Mbandaka – a city of a million people – about 130km from the area where the first cases were confirmed earlier this month.
The city is a major transportation hub with routes to the capital Kinshasa.
Forty-two people have now been infected and 23 people are known to have died.
The Ebola virus is a serious illness that causes internal bleeding and is often proven to be fatal. It can spread rapidly through contact with small amounts of bodily fluid and its early flu-like symptoms are not always obvious.
The spread to the city is a worry due to the fact that the 2014-2016 West Africa outbreak that killed 11,300 people was particularly deadly because it spread to the capital cities of Guinea, Sierra Leone and Liberia.
Senior World Health Organization (WHO) official Peter Salama said 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 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."
Mr Salama said that Mbandaka's location on the Congo River is widely used for transportation which raises the prospect of Ebola spreading to surrounding countries such as Congo-Brazzaville and the Central African Republic as well as downstream to Kinshasa, a city of 10 million people.
"This puts a whole different lens on this outbreak and gives us an increased urgency to move very quickly into Mbandaka to stop this new first sign of transmission," he said.
It is, however, been confirmed that suspected cases of Ebola have been recorded in three health zones of Congo's Equateur province, said the WHO.
Health workers had identified 430 people who may have had contact with the disease and were working to trace more than 4,000 contacts of Ebola patients, who have spread across northwest Congo, said the organisation.
Many of these people were in areas only reachable by motorbike, Mr Salama said.
On Wednesday more than 4,000 doses of an experimental vaccine, sent by the WHO, arrived in the country with another batch expected to reach the Congo soon.
The vaccine needs to be stored at a temperature of between -60 an -80ºC. Electricity supplies in Congo are unreliable.
"The logistic issues getting to this very remote area are quite considerable, and it will also be considered on the ground to identify who should be vaccinated and to get out in this vast and very difficult area and provide vaccination in an appropriate way," New York-based Ebola expert Dr Laurie Garrett told BBC.
"It's never been done before in the midst of an exploding outbreak so we'll watch it very closely."
The Ebola virus is thought to be spread over long distances by fruit bats and is often transmitted to humans via contaminated bushmeat.
The disease is endemic to the area and it is not possible to eradicate all the animals who might be a host for Ebola. As long as humans come in contact with them, there is always a possibility that Ebola could return.