The Red Cross announced on Saturday that three volunteers had died in the Democratic Republic of Congo after apparently contracting Ebola while on duty there in March.

The central African country and its neighboring Uganda have been gripped by an outbreak of the deadly viral disease, which the World Health Organization has declared an international public health emergency.

The International Federation of Red Cross and Red Crescent Societies said the volunteers were from the DR Congo Red Cross in Ituri, a northeastern province and the outbreak's epicenter. They worked for the Mongbwalu branch of the organization in Djugu territory, Ituri.

"Alikana Udumusi Augustin, Sezabo Katanabo and Ajiko Chandiru Viviane are believed to have contracted the Ebola virus on duty, while carrying out dead body management activities on March 27 as part of a humanitarian mission unrelated to Ebola," the IFRC said in a statement.

"At the time of the intervention, the community was not aware of the Ebola virus disease outbreak, and the outbreak had not yet been identified. They are among the first known victims of the outbreak."

The IFRC said one had died on May 5 and the other two on May 15 and 16.

"These volunteers lost their lives while serving their communities with courage and humanity," the Geneva-based IFRC said. "Their commitment reflects the extraordinary dedication shown every day by Red Cross volunteers working in complex and high-risk environments to support vulnerable people."

The IFRC, which has more than 17 million volunteers in more than 191 countries, is the world's largest humanitarian network. It said it remained committed to supporting communities affected by Ebola and to strengthening efforts to respond to the outbreak.

The Africa Centres for Disease Control and Prevention on Saturday increased the number of high-risk countries to include Angola, Burundi, the Central African Republic, Ethiopia, Kenya, Rwanda, South Sudan, Tanzania and Zambia.

In Uganda, authorities confirmed three new Ebola cases, bringing the total number of confirmed cases in the country up to five. The new cases include a Ugandan driver who transported the first confirmed case, a Ugandan health worker who was exposed while treating the first confirmed case and a Congolese woman who entered Uganda with mild abdominal symptoms.

There are 82 confirmed cases and seven confirmed deaths in the Congo, with almost 750 suspected cases and 177 suspected deaths, the WHO said.

Dr. Craig Spencer, an emergency room physician and public health professor at Brown University who survived Ebola after contracting it while working with Doctors Without Borders in Guinea in 2014, previously told CBS News that he was particularly worried about healthcare workers responding to the outbreak. 

"Healthcare workers are the group that I'm really concerned about because they had very close contact with people when they're most contagious, particularly around the time of folks' death," Spencer said. 

Ebola is a deadly viral disease spread through direct contact with bodily fluids. It can cause severe bleeding and organ failure. The strain of ebolavirus involved in the outbreak, Bundibugyo virus, is rarer and has no approved vaccine or treatments. 

The WHO on Friday raised the risk assessment for Congo to the highest level, from high to very high. The organization kept the regional risk level at high and the global risk level at low.