Nearly 1,400 people have died in the Democratic Republic of Congo.
Dr Jeremy Farrar, the director of the Wellcome Trust, said the epidemic was the worst since that of 2013-16 and has showed “no sign of stopping”.
Two people have also died in neighbouring Uganda, the first cases of Ebola reported in the country.
A five-year-old boy infected with the virus died on Tuesday and a woman died on Wednesday, the Ugandan health ministry said.
The Ugandan government has reported at least six other suspected cases of the virus.
In a statement, Dr Farrar said the spread was “tragic but unfortunately not surprising”. He warned that more cases were expected, and a “full” national and international response would be needed to protect lives.
“The DRC should not have to face this alone,” he said
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