The United Nations needs $4.4bn by the end of next month to prevent "a catastrophe" of hunger and famine in South Sudan, Nigeria, Somalia and Yemen, according to Secretary-General Antonio Guterres.

More than 20 million people face starvation in the four countries and action is needed now to avert a humanitarian disaster, Guterres told a news conference at UN headquarters on Wednesday.

"We need $4.4 billion by the end of March to avert a catastrophe," he said.

So far, the UN has raised just $90m.

South Sudan on Monday declared a famine in northern Unity State while Fews Net, the famine early warning system, has said that some remote areas of northeast Nigeria are already affected by starvation since late last year.

The four famine alerts are unprecedented in recent decades.

There has only been one famine since 2000, in Somalia. At least 260,000 people died in that disaster - half of them children under the age of five, according to the UN World Food Programme.

The UN children's agency UNICEF this week said almost 1.4 million children acutely malnourished in Nigeria, Somalia, South Sudan and Yemen could die from famine in the coming months.

READ MORE: UN demands action as famine looms in three countries

Of the four famine alerts, only one - Somalia - is caused by drought, while the other three are the result of conflicts, also described as "man-made food crises".

"The situation is dire," said Guterres.

"Millions of people are barely surviving in the space between malnutrition and death, vulnerable to diseases and outbreaks, forced to kill their animals for food and eat the grain they saved for next year's seeds."

The appeal for international action came as humanitarian aid groups are already struggling to meet needs in Syria and cope with the global refugee crisis.