An Ebola outbreak that has killed more than 330 people in West Africa is "totally out of control", Doctors Without Borders (DWB) officials said.
The deadly disease started in Guinea's Guekedou region in February and has since spread to Sierra Leone and Liberia infecting some 500 people.
The outbreak appeared to slow after an initial peak but health officials have since recorded a strong return for the killer virus in recent weeks.
"The reality is clear that the epidemic is now in a second wave," said DWB director of operations in Brussels Bart Janssens. "It is totally out of control."
Fourteen deaths and 47 new cases have been reported in the region in just the last week, according to the World Health Organisation (WHO).
"I'm absolutely convinced that this epidemic is far from over and will continue to kill a considerable amount of people, so this will definitely end up the biggest ever," Janssens said.
International organisations and governments are struggling to contain the disease which has, for the first time on record, affected multiple locations across several countries.
Earlier this week, WHO spokeswoman Fadela Chaib said they were facing one of the "most challenging Ebola outbreaks ever".
The virus struck a densely populated area where people are very mobile, experts said.
Health officials said there is an urgent need to increase public awareness urging people to come forward as soon as symptoms appeared.
"There needs to be a real political commitment that this is a very big emergency," Janssens said. "Otherwise, it will continue to spread, and for sure it will spread to more countries."
There is no cure or vaccine for Ebola, which kills up to 90% of those infected.
It is spread by close contact with symptoms including internal and external bleeding, diarrhoea and vomiting.
The WHO said 337 people have so far been killed by the virus in the region. The worst affected country was Guinea is with 264 Ebola-related deaths, followed by In Sierra Leone with 49 and Liberia where 24 people have died.