The head of the World Health Organization said he’s “deeply concerned about the scale and speed” of the Ebola outbreak sweeping through the Democratic Republic of Congo and Uganda after the death toll climbed above 100 people early this week.

At least 131 people have died and 531 cases are suspected in the outbreak, according to Congolese health authorities. The outbreak has alarmed health officials around the world because the rare Ebola strain involved, known as the Bundibugyo virus, has no known vaccine or treatment.