WFP's West Africa Director Denise Brown said the organization was currently providing food for around 150,000 people in Ebola-striken nations but needed to rapidly scale that up as the worst ever epidemic of the virus advanced.
Senegal on Friday became the fifth country to confirm it had been touched by the outbreak that has infected more than 3,000 people - killing some 1,550 of them - since it was detected in March. The World Health Organization (WHO) said on Thursday the outbreak could infect a total of 20,000 people before it ends.
Guinea, Liberia and Sierra Leone have pledged to impose a 'cordon sanitaire' on the most affected communities in their joint border region, restricting travel to and from the areas and limiting their access to food supplies.
"We need $70 million. That's for 1.3 million people for three months," Brown told Reuters late on Friday. "We've agreed this morning...that we need to extend that because WHO is already talking about 6-9 months before this is contained."
Brown said the WFP would look from donations from major donors like the United States, the European Union, the World Bank and Japan, as well as from non-traditional benefactors such as Arab states.
She warned, however, that the agency's resources were already thinly stretched by major humanitarian crises in Syria, Iraq, South Sudan and Central African Republic.
"I don't think the world has ever seen so many concurrent crisis on such a huge scale. The humanitarian community is stretched beyond belief," she said.
Brown said WFP started food distribution in Guinea around 4 months ago, and more recently in Liberia and Sierra Leone, mostly delivering food to isolation wards in hospitals before gradually increasing the scope of the mission.
Travel restrictions imposed by neighboring African countries, notably Senegal - a regional hub for the humanitarian sector - had made it more difficult to get staff and supplies into the affected region, Brown said.
The operation was also made more challenging by precautions to stop the disease spreading and staff becoming infected.
"We don't want to go in and do a distribution for 10,000 people. We want small groups of people, which is going to be very hard for us to manage," Brown said. "Yes, it probably makes us a bit slower but we need to get this right."
The area of Liberia hardest-hit around the northern Lofa county include some of its main food producing regions and the quarantine imposed on this area has raised fears that supplies to the rest of the country will be restricted.
Brown said that prices for rice and cassava at one of the main markets in the capital Monrovia had already risen by around 30 percent and there were reports that farmers had not been able to plant their crops because of contagion fears, suggesting shortages were likely to worsen.