More than one million children have fled South Sudan due to escalating conflict, the United Nations said.

Children make up 62% of the 1.8 million people who have fled South Sudan for refugee camps in neighbouring Ethiopia, Kenya and Uganda since civil war began in 2013, the UN children's agency, UNICEF, and refugee agency, UNHCR, said in a joint press release said Monday.

Another 1.4 million children are reportedly displaced within the country.

"The future of a generation is truly on the brink," said UNICEF's Leila Pakkala.

"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," she said.

"No refugee crisis today worries me more than South Sudan," said Valentin Tapsoba of the UNHCR.

Tens of thousands of people have been killed in the ongoing conflict, among them more than 1,000 children, the UN added.