At least 89 boys have been abducted in South Sudan’s restive Upper Nile state, the United Nations Children’s Fund (UNICEF) said Saturday.

The U.N. agency said the boys, some as young as 13, were taking exams in Wau Shilluk, a town about 12 miles northeast of the state capital, Malakal, when they were taken by armed men. UNICEF also cited witnesses to the mass abduction who said “armed soldiers” went house to house in the town and forced any boys over the age of 12 to go with them.

Thousands of children have been forced by the government and rebel groups to fight as child soldiers since South Sudan erupted in violence in December 2013, U.N. Secretary-General Ban Ki-moon said in a report released last month.

It is unclear who was behind the abductions in Wau Shilluk, but UNICEF’s representative in South Sudan, Jonathan Veitch, called for the children to be released immediately.

“The recruitment and use of children by armed forces destroys families and communities. Children are exposed to incomprehensible levels of violence. They lose their families and their chance to go to school,” Veitch said.

South Sudan seceded from Sudan in 2011 after decades of war. The new nation has been in conflict since December 2013, when President Salva Kiir accused his former deputy, Riek Machar, of trying to oust him in a coup. Their rivalry has sparked violence throughout the young nation, with reports of mass killings and starvation. More than 1.5 million people have been displaced.

•Text courtesy of VOA. Photo shows South Sudan’s President Salva Kiir.