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 Liu said the situation was "deteriorating faster, and moving faster, than we can respond to", the BBC reports.
Earlier, the World Health Organization (WHO) said the scale of the outbreak appeared to be "vastly underestimated" and that "extraordinary measures" were needed.
The epidemic began in Guinea in February and has since spread to Liberia, Sierra Leone and Nigeria.
On Friday, the death toll rose to 1,145 after WHO said 76 new deaths had been reported in the two days to 13 August. There have been 2,127 cases reported.
Ms Liu said that although Guinea was the initial epicentre, the pace there had slowed, and other countries - particularly Liberia - were now the focus.
"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."
Ms Liu also called for more action from the international community and stronger leadership from WHO - the UN's health agency.
"WHO needs to take leadership and bring some strong elements into the field at all operational levels. It's already started but it needs to happen at all levels."
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 - which declared a global health emergency last week - 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.
Meanwhile, athletes from Ebola-hit countries in west Africa have been barred from competing in some sports at the Youth Olympics that open in China this weekend.
The International Olympic Committee and Chinese organisers say that athletes from affected areas will not compete in combat sports, while it was also decided that no athletes from the region would compete in the pool.
The Games start on Saturday in Nanjing, but the ruling will only impact three athletes following the decision of Sierra Leone and Nigeria to withdraw.
Athletes will undergo regular checks.