The world has as little as three months to save millions of people from famine in Yemen and Somalia, the International Committee of the Red Cross said on Wednesday.

The medical charity needs $300m to bring emergency assistance to a total of five million people in Yemen, Somalia and northeast Nigeria, as well as areas of South Sudan where famine has already been declared.

The funds will ensure that five million of the 20 million people at risk of famine and starvation receive immediate and essential aid, said Red Cross director of operations Dominik Stillhart.

"Food, water, shelter, and healthcare is required immediately," he said.

"We are on the ground and delivering aid in all four countries. We witness the massive suffering. Millions of people are denied the very basics to survive."

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Earlier this month, the United Nations announced more than 20 million people were facing famine in the four countries, and it needs to raise $4.4bn in funds by July.

In South Sudan, the UN said about 3.4 million are in need of help after being displaced by fighting between government forces and armed groups loyal to the former vice president Riek Machar.

While several districts in South Sudan are already facing famine, Stillhart said Yemen and Somalia have a three-to-four-month "window" before it sets in there.

About 300,000 people, meanwhile, face acute malnutrition in northeast Nigeria.

UN and food organisations define famine as when more than 30 percent of children under the age of five suffer from acute malnutrition and mortality rates are two or more deaths per 10,000 people each day, among other criteria.

Stillhart said that aside from addressing the immediate threat of starvation, the international community must also tackle "the root causes of the crisis".