The Ebola outbreak in Congo has spread from the countryside into a city, prompting fears that the disease will be increasingly difficult to control.
Health Minister Oly Ilunga Kalenga confirmed a case in Mbandaka, a city of a million people about 130km (80 miles) 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.
Confirmed, probable and suspected cases of Ebola have been recorded in three health zones of Congo's Equateur province, the World Health Organisation (WHO) said.
The WHO's Peter Salama said 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 had spread across northwest Congo.
On Wednesday more than 4,000 doses of an experimental vaccine sent by the World Health Organisation (WHO) arrived in the country with another batch expected soon.
The vaccine from pharmaceutical firm Merck is unlicensed but was effective in limited trials during the Ebola outbreak in West Africa.
It needs to be stored at a temperature of between -60 and -80 C. Electricity supplies in Congo are unreliable.
Some 11,300 people died of Ebola in Guinea, Liberia and Sierra Leone between 2014 and 2016.