south sudan war child soldiers
Government and opposition forces in South Sudan are forcibly recruiting boys as young as 13 as child soldiersGetty

An armed group has abducted at least 89 boys who were sitting their school exams in South Sudan, the United Nations Children's Fund said on Saturday.

The boys were abducted near in Wau Shilluk near the capital Malakal, Unicef said. Some of the boys were as young as 13.

According to a Unicef statement, the children were abducted by an unidentified group of armed men, who went door to door and forcibly took boys older than 12.

Six teachers were also taken during the attack on 15 February, while the number of children who were abducted "could be much higher", said the chairty.

Jonathan Veitch, Unicef's representative in South Sudan, called for the immediate release of the children.

"Unicef reminds all parties involved in the conflict that the recruitment and use of children in armed forces and groups is a grave violation of international law," Veitch said. "Children are exposed to incomprehensible levels of violence, they lose their families and their chance to go to school."

The South Sudan government has denied a recent report by Human Rights Watch (HRW), which alleges that it has been recruiting boys as young as 13 to fight in the country's civil war, which has claimed thousands of lives.

"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," said Daniel Bekele, Africa director at Human Rights Watch. "In Malakal, government forces are even taking children from right outside the United Nations compound."

The South Sudan conflict erupted in December 2013 when President Salva Kiir, from the Dinka ethnic group, accused then-Prime Minister Riek Machar of plotting to overthrow the regime.

The accusations sparked violence in the country, where factions loyal to Kiir and Machar engaged in tit-for-tat violence that has claimed thousands of lives.

At least 4 million people are also at risk of a man-made famine, due to lack of funds and widespread fighting which has stopped volunteers reaching certain areas.