The international group Doctors Without Borders warned Tuesday that the world is 'losing the battle' against Ebola, while U.N. officials implored all countries to quickly step up their response by contributing health experts and other help.

Doctors Without Borders President Joanne Liu said her organization is completely overwhelmed as it treats Ebola patients in four West African countries. She called on countries with biological disaster response capacity to contribute civilian and military medical personnel.

"Six months into the worse Ebola epidemic in history, the world is losing the battle to contain it," Liu said at a United Nations forum on the outbreak. "Ebola treatment centres are reduced to places where people go to die alone, where little more than palliative care is offered."

World Health Organization Director Margaret Chan said the United Nations is urgently intensifying its response and called on all countries to contribute. She warned the outbreak would "get worse before it will get better and requires a global surge and scaled-up response."

She thanked countries that have helped but said, "we need more from you. And we also need those countries that have not come on board."

David Nabarro, who is co-ordinating the U.N. response, says the world body is "bringing in outside health workers as much as we can."

But Ameerah Haq, head of U.N. peacekeeping's Department of Field Support, warned that before bringing in external expertise, guarantees are needed for medical evacuations and treatment for any workers who become infected. Haq said "without one, the other will not happen."

The Ebola outbreak in West Africa has killed more than 1,500 people in Guinea, Liberia, Sierra Leone and Nigeria.

Earlier Tuesday, the U.N. Food and Agriculture Organization warned that food in countries hit by Ebola is becoming more expensive and will become scarcer as farmers can't reach their fields.

Authorities have cordoned off entire towns in an effort to halt the virus' spread. Surrounding countries have closed land borders, airlines have suspended flights to and from the affected countries and seaports are losing traffic, restricting food imports to the hardest-hit countries. Those countries — Guinea, Liberia and Sierra Leone — all rely on grain from abroad to feed their people, according to the U.N. FAO.