South Sudanese rebels on Tuesday released 280 child soldiers, the first batch of some 3,000 to be freed but with thousands more still fighting, the UN children's agency said.
Some 12,000 children, mainly boys, have been forcibly recruited by armed groups across the country in the past year alone to fight, according to UNICEF.
Those freed included some as young as 11, who had been fighting for up to four years and have never attended school.
"The first group of 280 children were released today, at the village of Gumuruk in Jonglei state" in the east of the country, UNICEF said in a statement, calling it "one of the largest ever demobilisations of children".
The remaining children will be released in the weeks ahead.
South Sudan, the world's youngest nation, has been locked in civil war since December 2013 when President Salva Kiir accused his sacked deputy Riek Machar of attempting a coup.
The fighting in the capital Juba set off a cycle of retaliatory massacres across the country.
"These children have been forced to do and see things no child should ever experience," UNICEF chief in South Sudan Jonathan Veitch said.
The children were in a rebel force led by David Yau Yau, who heads the South Sudan Democratic Army (SSDA) Cobra Faction insurgents, a small forced based in the Pibor region of Jonglei.
Yau Yau launched his rebellion in 2010 after losing out on a seat in elections, a year before South Sudan became independent from Sudan.
Children freed are being health checks, food, clothing and education.
The UN is trying to reunify them with their families, which it called a "daunting task" in a country where over two million have fled their homes to escape fighting.
Over a half million of those people are now refugees in neighbouring nations.
South Sudan's rival leaders are to meet later this week in Ethiopia in the latest push by regional nations to enforce a ceasefire already agreed -- and broken -- six times.