UNICEF had previously said at least 89 boys, some as young as 13, were forcibly recruited in mid-February by an armed group near the town of Malakal, the capital of the oil-producing Unity state.
UNICEF said in a statement today that it was "confident that the armed group which took the children ... Is aligned with" South Sudan's military.
Jonathan Veitch, the UNICEF representative in South Sudan, said he feared the children "are going from the classroom to the front line."