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.

The price of cassava root, a staple in many West African diets, has gone up 150 per cent in one market in Liberia's capital, Monrovia.

"Even prior to the Ebola outbreak, households in some of the affected areas were spending up to 80 per cent of their incomes on food," said Vincent Martin, who is co-ordinating the food agency's response to the crisis. "Now these latest price spikes are effectively putting food completely out of their reach."

An estimated 1.3 million people in Guinea, Liberia and Sierra Leone will soon need help feeding themselves, said the U.N.

Chan and other officials at the U.N. forum criticized the border closures because they are preventing supplies from reaching people in desperate need.

"The three worst-hit countries are isolated," Chan said. "We cannot fly in our experts for help."

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The situation will likely worsen because restrictions on movement are preventing labourers from getting to farms and the harvest of rice and corn is set to begin in just a few weeks, the FAO said.

Ivory Coast decided Monday night to keep its borders with Guinea and Liberia closed but said it would open a humanitarian corridor to allow supplies in.

A separate Ebola outbreak has hit a remote part of Congo, in Central Africa, the traditional home of the disease. So far, 53 cases consistent with Ebola have been identified there, of whom 31 have died, WHO said Tuesday.