GENEVA, Switzerland: The World Health Organization said on May 20 that the Ebola outbreak in eastern Democratic Republic of Congo likely began about two months ago and is expected to continue expanding.

The outbreak involves the rare Bundibugyo strain of Ebola, for which there is no approved vaccine or virus-specific treatment. Health officials say the delay in detecting the outbreak has complicated efforts to trace contacts and contain the disease.

Anais Legand, a technical officer for viral threats at the World Health Organization, said investigators are still working to determine the exact origin of the outbreak.

"Investigations are ongoing to ascertain when and where exactly this outbreak started," Legand told reporters in Geneva. "Given the scale, we are thinking that it has started probably a couple of months ago."

World Health Organization Director-General Tedros Adhanom Ghebreyesus said 600 suspected cases and 139 suspected deaths have been reported so far. Laboratory tests have confirmed 51 cases in the Democratic Republic of Congo and two additional cases in neighboring Uganda.

Tedros said a WHO Emergency Committee met on May 19 and confirmed that the outbreak constitutes a public health emergency of international concern, but not a pandemic emergency.

"The WHO assesses the risk of the epidemic as high at the national and regional levels and low at the global level," Tedros said.

The Bundibugyo strain has an average fatality rate of about 40 percent, according to the WHO.

Chikwe Ihekweazu, executive director of the WHO Health Emergencies Program, said the immediate focus is to identify all transmission chains.

"Our absolute priority now is to identify all the existing chains of transmission ... that will then enable us to really define the scale of the outbreak and be able to provide care," Ihekweazu said.

Health officials suspect the outbreak accelerated after a super-spreading event, possibly at a funeral or healthcare facility.

The WHO said two potential vaccines are under consideration, but they could take three to nine months to become available.

Among the confirmed cases is an American doctor who was working in Congo and has been transferred to Germany. Another U.S. doctor exposed to an Ebola patient in Uganda is being transported to a hospital in Prague, according to Czech officials.

The U.S. government has committed an initial US$13 million to support the response and plans to help open 50 treatment clinics. The Africa Centers for Disease Control and Prevention said South Africa has pledged an additional $2.5 million.