The head of a major medical research charity has called the latest outbreak of Ebola in central Africa "truly frightening".
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".
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.
Since the first case of Ebola in the DRC last August, nearly 1,400 people have died – around 70 per cent of all those infected.
The outbreak is the second-largest in the history of the disease, with a significant spike in new cases in recent weeks.
Only once before has an outbreak continued to grow more than eight months after it began – that was the epidemic in West Africa between 2013-16, which killed 11,310 people.
Efforts to contain the spread have been hindered by militia group violence and by suspicion towards foreign medical assistance.
Nearly 200 health facilities have been attacked in the DRC this year, forcing health workers to suspend or delay vaccinations and treatments. In February, medical charity Médecins Sans Frontières (MSF) put its activities on hold in Butembo and Katwa – two eastern cities in the outbreak's epicentre.
In Uganda, a five-year-old boy died of the virus on Tuesday, according to the World Health Organisation (WHO).
His grandmother died Wednesday while his younger brother also had the disease.
The boy is said to have travelled across the border with his family from the DRC on Sunday. He was then taken to a Ugandan hospital after exhibiting symptoms, including vomiting blood, officials said.
Seven other cases have been confirmed in the country, and Uganda's government said 50 people were suspected to have come into contact with those infected.
This outbreak is already the second largest in human history and some have predicted it could take up to two more years to bring to an end.
The World Health Organization has twice ruled that this Ebola outbreak is not yet a global emergency. Its Emergency Committee will meet again on Friday.
In Uganda, mass gatherings including market days and prayers have been cancelled. Market days in the town of Kasese attract an estimated 20,000 people at the border area.
Uganda's health ministry and the WHO said a rapid response team had been dispatched to identify others at risk.
The country has already vaccinated about 4,700 health workers against the disease, according to a joint statement by WHO and Ugandan health officials.