An unidentified South Sudan armed group has abducted at least 89 boys, some as young as 13, from their homes in the north of the country, UNICEF said Saturday.
"Eighty-nine children were abducted ...," a statement said, adding that "the actual number could be much higher."
The UN children's agency said the mass abduction happened at the start of the week in the town of Wau Shilluk.
Witnesses said that unidentified armed soldiers surrounded the community and went house-to-house taking away by force any boys thought to be over 12 years old.
"The recruitment and use of children by armed forces destroys families and communities," said Jonathan Veitch, the head of UNICEF in South Sudan.
"Children are exposed to incomprehensible levels of violence, they lose their families and their chance to go to school."
UNICEF estimates there are at least 12,000 children used by both sides in South Sudan's ongoing civil war.
Recruitment of children has increased since fighting began in December 2013 when President Salva Kiir accused his former deputy, Riek Machar, of planning a coup.
War continues despite numerous ceasefire deals and with peace talks underway in the Ethiopian capital, Addis Ababa.
Earlier this month advocacy group Human Rights Watch accused both rebel and government forces of "actively recruiting" child soldiers despite national laws prohibiting it and repeated promises to stop the practice.
Information minister Michael Makuei dismissed the report, saying the government did not use children in combat as there were plenty of men able to fight.
It was not clear which armed group was responsible for this week's mass abduction in Wau Shilluk, a riverside town in government-held territory within Upper Nile state.
It has grown dramatically with the arrival of tens of thousands of people forced from their homes during 14-months of war, many fleeing from the nearby city of Malakal, where fighting has been particularly fierce.
The area is under the control of government-aligned warlord Johnson Olony, who HRW accused of recruiting children in its February report.