UPDATE 1-WHO extremely concerned about Ebola "perfect storm" in Congo

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

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

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

The response was 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, there were major obstacles ahead, Salama said.

Attacks by armed opposition groups had 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. On Monday 80 percent of Ebola contacts and three suspected cases in and around Beni could not be reached for disease monitoring.

A "contact" is a person at risk of developing the disease because they may have had contact with it, so they require monitoring until they are out of a 21 day incubation period.

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