The outbreak of Ebola in West Africa will take at least six months to bring under control, the medical charity Medecins Sans Frontieres (MSF) says.
Speaking in Geneva, MSF President Joanne Lui called for strengthened international co-ordination led by the World Health Organization (WHO).
Earlier, WHO said the scale of the outbreak appeared to be "vastly underestimated".
It said that "extraordinary measures" were needed.
The epidemic began in Guinea in February and has since spread to Liberia, Sierra Leone and Nigeria.
Lui said that controlling the outbreak in Liberia - which has recorded more than 300 deaths - was vital to containing the epidemic.
"If we don't stabilise Liberia, we will never stabilise the region," she said.
"In terms of timeline, we're not talking in terms of weeks, we're talking in terms of months. We need a commitment for months, at least I would say six months, and I'm being, I would say, very optimistic."
Lui also called for more action from the international community, led by WHO - the UN's health agency.
"All governments must act. It must be done now if we want to contain this epidemic," she said.
Ebola is transmitted by direct contact with the body fluids of a person who is infected.
Initial flu-like symptoms can lead to external haemorrhaging from areas such as eyes and gums, and internal bleeding which can lead to organ failure.
The WHO recently said that the risk of transmission of Ebola during air travel remained low, as the disease is not airborne.
As a consequence, Kenya Airways has rejected pressure to suspend its flights to the Ebola-hit states of West Africa.
The WHO said in a statement that its staff had seen evidence that the number of reported cases and deaths did not reflect the scale of the crisis.
WHO spokesman Gregory Hartl said that experts going house-to-house in Kenema, Sierra Leone, in search of infected people were discovering more cases.
"We have seen with the opening of new treatment centres, many more Ebola patients come forward, and so this has given rise to the belief that there are probably a lot of patients out there that we haven't traced or contacted yet," he said.
He said that an 80-bed treatment centre that recently opened in Liberia's capital Monrovia filled up immediately and that the next day, dozens more people turned up to be treated.
Tarnue Karbbar, of the aid group Plan International in northern Liberia, said medical teams were not able to document all the cases erupting.
He said many of the sick were being hidden at home by relatives, and many victims were buried before teams could get to the area.
The outbreak is also affecting the Youth Olympic Games about to start in China. The International Olympic Committee (IOC) has ruled that athletes from Ebola-hit countries will not be allowed to compete in combat
sports or in the pool, and Sierra Leone and Nigeria have withdrawn from the Games.