An Ebola outbreak in West Africa has killed more than 1,500 people, and authorities have cordoned off entire towns to try to halt its spread.
Neighbouring countries have closed land borders, airlines have suspended flights to and from affected countries and seaports are seeing less traffic, restricting food imports to the hardest-hit nations. Those countries - Guinea, Liberia and Sierra Leone - rely on grain imports to feed the population, said the UN Food and Agriculture Organisation.
Vincent Martin, who is co-ordinating the agency's response to the crisis, said: "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. Now these latest price spikes are putting food out of their reach."
The UN has said that 1.3 million people in Guinea, Liberia and Sierra Leone will need help to feed themselves in coming months.
The situation looks likely to worsen, the UN said, because restrictions on movement are preventing labourers from accessing farms. The harvest of rice and corn is due to begin in a few weeks.
The World Health Organisation (WHO) is asking countries to lift border closures because they are preventing supplies from reaching people in desperate need.
Ivory Coast has decided to keep its borders with Guinea and Liberia closed but said it would open a humanitarian corridor to allow supplies in.
The head of medical charity Medecins Sans Frontieres said the spread of the virus could not be stopped unless wealthy nations sent specialised teams to the affected countries. Joanne Liu added: "Six months into the worst Ebola epidemic in history, the world is losing the battle to contain it."
She said the countries did not have the capacity to cope with the outbreak, which has killed more than 1,500 people.
The death toll from a separate Ebola outbreak in the Djera region of Democratic Republic of Congo has risen to 31.