GENEVA (Reuters) - The World Health Organization (WHO) said on Tuesday an Ebola outbreak in northeastern Democratic Republic of Congo could worsen rapidly because of attacks by armed groups, community resistance and the geographic spread of the disease.

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At least 100 people have died in the outbreak, out of 150 cases in North Kivu and Ituri provinces.

“We are now extremely concerned that several factors may be coming together over the next weeks and months to create a potential perfect storm,” the WHO’s head of emergency response, Peter Salama, told a news conference in Geneva.

The response is at a critical juncture and, although the weekly number of new cases has fallen from about 40 to about 10 in the past few weeks and more than 11,700 people have been vaccinated, major obstacles remain ahead, Salama said.

Attacks by armed opposition groups have increased in severity and frequency, especially those attributed to the Alliance of Democratic Forces, most dramatically an attack that killed 21 in the city of Beni, where WHO’s operation is based.

The city has declared a “ville morte”, a period of mourning until at least Friday, obliging WHO to suspend its operations.

Overnight on Monday, unidentified assailants entered the town of Oïcha, about 20 km (12 miles) north of Beni, burned houses, killed one man and kidnapped 14 children and one woman, according to two local officials. Oicha has two confirmed cases of the virus and one probable case.

On Monday 80 percent of Ebola contacts -- people at risk of developing the disease and so requiring monitoring -- and three suspected cases in and around Beni could not be reached for disease monitoring.

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Pockets of “reluctance, refusal and resistance” to accept Ebola vaccination were generating many of the new cases, Salama said.

“We also see a very concerning trend. That resistance, driven by quite natural fear of this terrifying disease, is starting to be exploited by local politicians, and we’re very concerned in the run up to elections, projected for December, that exploitation... will gather momentum and make it very difficult to root out the last cases of Ebola.”

Some people were fleeing into the forest to escape Ebola follow-up treatment and checks, sometimes moving hundreds of kilometers, he said.

There was one such case to the south of Beni, and another to the north, close to the riverbanks of Lake Albert. Both were inaccessible for security reasons.

Neighboring Uganda was now facing an “imminent threat”, and social media posts were conflating Ebola with criticism of the DRC government and the United Nations and “a range of conspiracy theories”, which could put healthworkers at risk.

“We will not yet consider the need to evacuate but we are developing a range of contingency plans to see where our staff are best located,” he said.

“If WHO and its partners had to leave North Kivu ... we would have grave concerns that this outbreak would not be able to be well controlled in the coming weeks or months.”