WAU SHILUK, South Sudan — At the beginning of the rainy season every year, Nyaaker Onwar, 34, would plant the sorghum and vegetables, while her husband and eldest son herded the cows and caught fish from the White Nile. They ate what they produced, and when the harvest was bountiful, they sold the rest in a nearby market town.

This year, things are different.

In February, armed men looted their cows, burned their fishing boat and kidnapped some of their relatives. Ms. Onwar fled with her husband and seven children to this village through rain and deep mud. She has been here for weeks with thousands of people displaced by South Sudan’s civil war. When she arrived, she was hungry, with no money and few options.

“We had to sell our clothes to buy food,” she said.

Five months of war in South Sudan has led to the deaths of thousands and the displacement of more than one million people. But officials warn that the tragedy could just be beginning. A serious food crisis is looming over the country, and the United Nations says that if action is not taken immediately, the consequences could be dire.

“There is every likelihood that the worst food crisis in South Sudan’s history can happen,” said Hilde Johnson, chief of the United Nations mission in South Sudan. “This can involve a famine of significant proportions.”