More than 1 million children have fled South Sudan's civil war, two UN agencies said Monday, part of the world's fastest growing refugee crisis.

Another 1 million South Sudanese children are displaced within the country, having fled their homes due to the civil war, said the UN's child and refugee agencies in a statement Monday.

"The future of a generation is truly on the brink," said Leila Pakkala, Unicef's Regional Director for Eastern and Southern Africa. "The horrifying fact that nearly one in five children in South Sudan has been forced to flee their home illustrates how devastating this conflict has been for the country's most vulnerable."

Roughly 62 per cent of refugees from South Sudan are children, according to the UN statement, and more than 75,000 children are alone or without their families. Roughly 1.8 million people have fled South Sudan in total.

"No refugee crisis today worries me more than South Sudan," said Valentin Tapsoba, UNHCR's Africa Bureau Director. "That refugee children are becoming the defining face of this emergency is incredibly troubling."

For children still living in South Sudan, the situation is still grim. Nearly three quarters of children are out of school, according to the UN statement, which is the highest out-of-school population in the world. An official famine was declared in two counties of South Sudan in February, and hundreds of thousands of children are at risk of starvation in the absence of food aid, according to the UN.

More than one thousand children have been killed or wounded in the East African nation's civil war. Both sides have pledged not to recruit child soldiers, but have ignored their promises. A UN official, who spoke on the condition of anonymity, said that opposition groups are recruiting inside UN displacement sites. The official spoke on the condition of anonymity because of lack of authorisation to speak to the media.

South Sudan's civil war began in December 2013 and has killed tens of thousands of people. South Sudan is now the world's fastest growing refugee crisis.