Uganda announced two more cases of Ebola on Wednesday - confirmation of the first spread of a deadly outbreak beyond the borders of Democratic Republic of Congo.
The Ugandan cases showed the epidemic was entering a "truly frightening" phase and was likely to spread further and kill many more people, one infectious disease specialist told Reuters.
Emergency medical teams and Uganda's health minister, Jane Ruth Aceng, were travelling to the border area where a five-year-old boy who contracted Ebola died late on Tuesday.
He had just crossed the border from Congo with his family who were now being monitored in isolation, a Ugandan health official said.
"Two more samples ... have tested positive," the World Health Organization agency said on Twitter, citing the health minister and bringing the total tally of confirmed cases to three.
"This epidemic is in a truly frightening phase and shows no sign of stopping anytime soon," said Jeremy Farrar, an infectious disease specialist and director of the Wellcome Trust global health charity which is involved in fighting Ebola.
"We can expect and should plan for more cases in DRC and neighbouring countries," he said, adding: "There are now more deaths than any other Ebola outbreak in history, bar the West Africa Epidemic of 2013-16, and there can be no doubt that the situation could escalate towards those terrible levels."
The current Ebola epidemic began in August last year in eastern Congo and has already infected at least 2,062 people, killing 1,390 of them.
The viral disease spreads through contact with bodily fluids, causing hemorrhagic fever with severe vomiting, diarrhoea and bleeding.
This outbreak is the second largest on record after a West Africa epidemic in 2013-2016 infected 28,000 people and killed 11,300, mostly in Liberia, Guinea and Sierra Leone.
Uganda's health minister was travelling to the Ugandan border town of Kasese where the boy had been treated, a health ministry spokesman said.
Kasese lies at the site of a copper mine in the foothills of the ice-capped Rwenzori mountains and traders frequently cross over into Congo.
Experts noted that Uganda, which has been on high alert for possible spread of Ebola and has already vaccinated many frontline healthworkers, is relatively well prepared and should be able to limit the virus' spread.
"The current cases in Uganda will be quickly contained but the failure to stop the current Ebola epidemic in DRC is simply tragic," said Ian Jones, a professor virology at Britain's Reading University.