Human Rights Watch (HRW) is urging the South Sudanese Government and opposition forces to stop using child soldiers.
Citing several cases, the watchdog claims that boys as young as 13 are being recruited by government forces as soldiers in Malakal, Upper Nile state.
Rebels are also allegedly using children in battles and for other duties such as cooking and carrying water and ammunition.
Both opposition and government forces made a commitment to the UN to stop recruiting and using children under 18 as combatants with immediate effect in May and June 2014, respectively.
Human Rights Watch Africa director Daniel Bekele said: "Despite renewed promises by both government and opposition forces that they will stop using child soldiers, both sides continue to recruit and use children in combat.
"In Malakal, government forces are even taking children from right outside the UN compound.
"South Sudanese children's lives are being devastated by conflict, with children once again going to war instead of to school.
"Both sides should stop recruiting children and hold those responsible to account."
Under the laws of war, the recruitment or use of children under 15 by parties to a conflict amounts to a war crime, for which commanders can be held criminally responsible.
According to Unicef, thousands of children have already fought in the war and a recent UN report documented the use and recruitment of 561 children since the beginning of the conflict.
HRW is urging the government to ratify the Optional Protocol to the Convention on the Rights of the Child on the involvement of children and armed conflict, which sets 18 as the minimum age for any participation in armed conflict.
In addition, the watchdog wants commanders to cooperate with relevant UN agencies to help children return to places of safety.
Last month, Unicef secured the release of nearly 3,000 child soldiers from South Sudanese Government and opposition forces, which is claimed to be one of the largest ever demobilisations of children in a conflict zone.
Image: Child soldiers gather in Gumuruk to hand over their weapons at a demobilisation ceremony in Jonglei State in eastern South Sudan. Photo: © 2015 Reuters.