The World Health Organization has declared the Ebola outbreak in the Democratic Republic of Congo and Uganda a public health emergency of international concern, warning that the rare strain of the virus, which currently has no approved vaccine or treatment, may be more widespread than detected.

The World Health Organization said on Sunday that the outbreak caused by the Bora virus has reached the highest alert level under the International Health Regulations due to cross-border transmission, unexplained clusters of deaths and significant uncertainty about the scale of the epidemic.

Previously, cases of Ebola were confirmed in Kampala, the capital of Uganda, and Kinshasa, the capital of Congo (DRC), a city with a population of about 20 million, confirming that the virus has spread beyond the remote mining areas where the outbreak was first detected.

"This event is extraordinary," the World Health Organization director-general said in a statement, citing continued unrest in eastern Congo due to the lack of an approved vaccine or treatment for this strain and evidence that the outbreak may be much larger than the official number of cases.

As of May 16, Congo has reported 8 laboratory-confirmed cases, 336 suspected infections and 87 suspected deaths in Ituri province, according to the Africa Centers for Disease Control and Prevention. Uganda has confirmed two cases and one death in Kampala, both in travelers arriving from Congo.