This is the ninth time Ebola has been recorded in the Democratic Republic of Congo since the disease made its first known appearance – near the vast central African country’s northern Ebola river – in the 1970s.

“One of the defining features of this epidemic is the fact that three health professionals have been affected,” Health Minister Oly Ilunga said in a statement.

“This situation worries us and requires an immediate and energetic response.”

Most of the cases so far have been recorded around the village of Ikoko Impenge, near the northwestern town of Bikoro.

“After contact, the nurses began showing signs … We have isolated them,” Serge Ngaleto, the director of Bikoro’s main hospital, told Reuters by phone.

Congo’s long experience of Ebola and its remote geography mean outbreaks are often localised and relatively easy to isolate.

But Ikoko Impenge and Bikoro are situated not far from the banks of the Congo River, a major artery for trade and transport upstream from the capital Kinshasa. The Congo Republic is just on the other side of the river.

A spokesman for the director of epidemiology in Congo Republic said government experts would meet on Thursday to discuss measures to prevent it crossing the border.

Nigeria’s immigration service said on Thursday it had increased screening tests at airports and other entry points as a precautionary measure.

Similar measures helped it contain the virus during the West African epidemic that began in 2013.

Officials in Guinea and Gambia both said they had heightened screening measures along their borders to prevent the spread.

Democratic Republic of Congo’s health ministry said it had dispatched a team of 12 experts to the northwest to try to trace new contacts of the disease, identify the epicentre and all affected villages and provide resources.