NAIROBI — Three cases of Ebola have been confirmed in western Uganda in the first indication during Congo’s year-long outbreak that the deadly virus is spreading outside the country.

The first case in Uganda was a 5-year-old Congolese boy who had crossed the border to seek medical treatment, the World Health Organization and Uganda’s Health Ministry said in a joint announcement Tuesday. WHO later reported he died overnight.

Authorities subsequently identified two other cases — family members of the deceased. His 3-year-old brother and 50-year-old grandmother have Ebola and have been isolated at a hospital near the border.

The boy and his family crossed the border Sunday, and the case was confirmed at a rural hospital. The boy was then put in isolated care, and “rapid response teams” have been sent to the surrounding area to follow up with potential contacts of the boy and his family.

“In preparation for a possible imported case during the current outbreak in DRC, Uganda has vaccinated nearly 4,700 health workers in 165 health facilities (including in the facility where the child is being cared for); disease monitoring has been intensified; and health workers trained on recognizing symptoms of the disease,” the joint statement said, using an abbreviated form of Congo’s formal name.

The outbreak has infected at least 2,062 people in Congo, and about 1,400 have died. Until Tuesday, it had been somewhat confined to a densely populated area in Congo’s North Kivu province. It is the worst outbreak of the virus since a massive epidemic swept through three West African countries from 2014 to 2016, killing more than 11,000.

The effort to contain it has been severely hampered by ongoing conflict in North Kivu that has sown distrust between outside organizations, such as the WHO and United Nations, and locals wary of interference in their affairs. Health workers have regularly come under attack.

Uganda accepts refugees from Congo, and many health workers felt that despite their best efforts, spread of the virus to Uganda had just been a matter of time.

“Our teams in Uganda have been training and preparing for this possibility, working in infection prevention and control and training health clinics in the country to recognize symptoms and safely triage and transfer suspected cases to Ebola treatment centers,” said Kellie Ryan, a spokeswoman for the International Rescue Committee, which has teams on the periphery of the outbreak zone, both in and outside of Congo, preparing for potential new cases.