A Congolese health worker prepares to administer Ebola vaccine. File picture: Olivia Acland/Reuters.

Geneva - The World Health Organization (WHO) is suspending parts of its effort to fight Ebola in the Congo after a string of attacks by militants, the UN agency said Tuesday.

Ebola has claimed 100 lives since the deadly virus outbreak started nearly two months ago in North Kivu, a north-eastern border region where more than 100 armed groups operate.

After 21 people died in an attack by the Allied Democratic Forces rebels in the city of Beni on Saturday, community groups called for days of mourning and protests that are expected to last at least until Friday.

WHO staff are confined to hotels and their operating centre in Beni for security reasons so their ability to track potential new infections has fallen sharply, WHO's emergency response chief Peter Salama told a press conference in Geneva.

"We are now extremely concerned that several factors may be coming together in the next weeks or months to create a potentially perfect storm," the Australian epidemiologist added.

In addition to the security risk, a minority of the local population is deeply mistrustful of government and UN health workers. Some potentially infected people have fled treatment.

Politicians in the region are also stoking the mistrust ahead of upcoming elections in December, Salama said Tuesday.

As a result, the outbreak has spread over a wider region including an area bordering Uganda.

The possibility of a cross-border-spread of Ebola has increased and Uganda "is facing an imminent threat," Salama added.