A surge in new Ebola cases has been reported in a densely populated Congolese city, adding a dangerous dimension to the worst outbreak of the virus in the country’s history.

Health workers have been forced to open up a new and particularly perilous front after 28 people died of Ebola in and around Butembo, a city of approximately 1.2 million people that is located in the heart of the country’s most volatile regions.

The government of the Democratic Republic of the Congo admitted this week that the country’s tenth Ebola outbreak is now officially its worst ever, after the number of confirmed infections crossed the 300 threshold. The virus has killed 211 people, the ministry of health said.

Unlike previous outbreaks, which have been restricted to fairly remote, rural areas, the present epidemic has swept through populated parts of northeastern Congo, a lawless area that is infested with rebel groups, freelance militias and armed criminal gangs.

Already struggling in conditions more challenging than previous outbreaks anywhere in Africa, health workers are now trying to deal with an outbreak that has multiple epicentres.

But nowhere is as worrying as Butembo, where a blend of insecurity, suspicion of aid workers and a large, mobile population have created an ideal landscape for the rapid spread of the disease.