A 24-year-old man in Uganda's central Mubende district showed symptoms and later died.
"We want to inform the country that we have an outbreak of Ebola which we confirmed yesterday," Diana Atwine, the health ministry's permanent secretary, told a news conference.
She said the patient with the confirmed case had high fever, diarrhoea and abdominal pains and was vomiting blood. He had initially been treated for malaria.
There are currently eight suspected cases receiving care in a health facility, WHO's Africa office said, adding that it was helping Uganda's health authorities with their investigation and deploying staff to the affected region.
"Uganda is no stranger to effective Ebola control. Thanks to its expertise, action has been taken to quickly detect the virus and we can bank on this knowledge to halt the spread of infections," Matshidiso Moeti, WHO Africa's regional director, said.
The WHO said there had been seven previous outbreaks of the Ebola Sudan strain, four in Uganda and three in Sudan.
It said Uganda last reported an outbreak of Ebola Sudan strain in 2012 and an outbreak of the Ebola Zaire strain in 2019.
The WHO said ring vaccination of high-risk people with the Ervebo vaccine had been highly effective in controlling the spread of Ebola in recent outbreaks in the Democratic Republic of the Congo and elsewhere but that this vaccine had only been approved to protect against the Zaire strain.
Another vaccine produced by Johnson & Johnson may be effective but has yet to be specifically tested against the Sudan strain, it added.
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