The number of Ebola cases and deaths in the Democratic Republic of Congo (DRC) and Uganda is expected to rise, the World Health Organization (WHO) has warned, following an outbreak linked to a rare strain of the virus.

Tedros Adhanom Ghebreyesus, director-general of the WHO, said Wednesday that 51 cases have been confirmed in Congo’s northern provinces of Ituri and North Kivu, “although we know the scale of the epidemic is much larger.”

He said Uganda has also reported two confirmed cases in the capital, Kampala. “Beyond the confirmed cases, there are almost 600 suspected cases and 139 suspected deaths,” he said. “We expect those numbers to keep increasing.”

There is a low risk of the outbreak spreading globally, he added, but a high risk of it spreading nationally and regionally in DRC and Uganda.

Tedros previously said he was “deeply concerned about the scale and speed” of the outbreak and declared it a public health emergency.

The U.S. Centers for Disease Control and Prevention confirmed Monday that an American doctor working in Congo had tested positive for Ebola and that six other U.S. citizens there have been exposed.

Satish Pillai, the CDC’s Ebola response incident manager, said Tuesday that the doctor was in transit to Germany for care. The transit plans for the six other individuals considered high-risk contacts to Europe are also being finalized.

Pillai added that the risk to the United States remains “low.”

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