Speaking in Geneva, MSF President, Joanne Lui, said the situation was “deteriorating faster, and moving faster, than we can respond to”.
Earlier, the World Health Organization (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.
So far, 1,069 people have died.
Ms Lui said that although Guinea was the initial epicentre, the pace there had slowed and other countries – particularly Liberia – now had to be the focus.
“If we don’t stabilise Liberia, we will never stabilise the region,” she said.
Health systems in West Africa are being severely strained by the Ebola outbreak
“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 Lui also called for more action from the international community and stronger leadership from WHO – the UN’s health agency.
“All governments must act. It must be done now if we want to contain this epidemic,” she said.
“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.