Almost 1.4 million children suffering from severe malnutrition could die this year as famine looms in Nigeria, Somalia and Yemen, the UN children's agency said.

The warning comes a day after government officials and the United Nations declared famine in parts of South Sudan.

In Yemen, where war has been raging for nearly two years, 462,000 children are suffering from acute malnutrition, while 450,000 children are severely malnourished in northeast Nigeria.

FEWS NET, the famine early warning system, said some remote areas of Nigeria's Borno state have been affected by famine since late last year.

The disaster is likely to continue, it said, as aid agencies are unable to reach those in need.

Drought in Somalia has left 185,000 children on the brink of famine but that figure is expected to reach 270,000 over the next few months, said UNICEF.

READ MORE: Famine declared in part of South Sudan's Unity state

In South Sudan, more than 270,000 children are malnourished and a famine has just been declared in parts of Unity State, in the north of the country, where 20,000 children live.

Aid agencies only describe a crisis as a famine when at least 20 percent of the population has access to fewer than 2,100 kilocalories of food a day and acute malnutrition affects more than 30 percent of the area's children.

Another reason to declare a famine is when there are two hunger-related deaths per 10,000 people, or four child deaths per 10,000 children every day.

UNICEF director Anthony Lake appealed for quick action.

"We can still save many lives," he said.

UN Security Council ambassadors are due to travel to northern Nigeria, Cameroon, Chad and Niger next month to draw international attention to the humanitarian crisis triggered by the conflict with Boko Haram fighters.