The head of a major medical research charity called the last outbreak in Central Africa to be "really frightening."
Nearly 1,400 people have died in the Democratic Republic of Congo.
Dr. Jeremy Farar, Director of Wellcome Trust, said the epidemic was the worst after 2013-2016 and showed "no signs of stopping".
A five-year-old boy also died in neighboring Uganda, the first Ebola case reported in the country.
The Ugandan government has already reported seven other suspected cases of the virus.
In a statement, Dr Farar said the spread was "tragic, but unfortunately not surprising." He warned that more cases are expected and that a "complete" national and international response to the protection of life will be needed.
"The DRC should not face this alone," he said.
What is the situation so far?
After the first Ebola case in the DRC in August last year, nearly 1,400 people died – about 70% of all infected.
The epidemic is the second largest in the history of the disease, with a significant leap in new cases in recent weeks.
Only once before, the epidemic has continued to grow more than eight months after the onset – the West African epidemic between 2013 and 1616, in which 11,111 people died.
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Efforts to curb the spread were hampered by militia violence and suspicion of foreign medical care.
Nearly 200 healthcare establishments have been attacked in the DRC this year, forcing health workers to stop or postpone vaccinations and treatment. In February, the medical charity Médecins Sans Frontières (MSF) withdrew its activities in Butembo and Katwa – two East European cities at the epicenter of the hearth.
In Uganda, a five-year-old boy died of the virus on Tuesday, according to the World Health Organization (WHO).
Officials said his grandmother and younger brother also had the disease. It is said that the boy had traveled across the border with his family from the DRC on Sunday. He was then taken to a hospital in Uganda after showing symptoms, including vomiting blood.
Seven other cases were confirmed in the country, and the Ugandan government said 50 people were suspected of contact with the infected.
Analysis by James Gallagher, Correspondent on Health and Science, BBC News
Ebola cases that occur in another country are always a significant and worrying development. The main question now is how much the virus has spread to Uganda?
Was there only the family who had crossed the border from the Democratic Republic of Congo or spread more widely? This is a moment for which Uganda has long been preparing, and I hope measures such as preventive vaccination of health workers will reduce the risk of embole spread.
This epidemic is now the second largest in human history and some predict that it may take another two years to end. The World Health Organization has twice ruled that this outbreak of Ebola is not yet global. The Emergency Committee will meet again on Friday.
What is being done to prevent the spread?
In Uganda, mass gatherings, including market days and prayers, were canceled. Shopping days in the city of Cassete attract about 20,000 people in the border area.
The Department of Health of Uganda and the WHO reported that a rapid response team was sent to identify risky others and to track eight possible cases.
The country has already vaccinated around 4,700 health workers against the disease, according to a joint statement by WHO and Ugandan health officials.
On Wednesday, WHO Head Tedros Adhanom Ghebreyesus said he would hold a meeting of the MME Emergency Committee on June 14th. The group will decide whether the outbreak should be considered exceptional for public health.
What is Ebola?
- Ebola is a virus that initially causes sudden fever, intense weakness, muscle aches and a sore throat.
- Progression to vomiting, diarrhea and internal and external bleeding.
- People get infected when they have direct contact through broken skin or mouth and nose, blood, vomiting, stools or body fluids of someone with Ebola.
- Patients tend to die from dehydration and multiple organ failure.