Story highlights South Sudan has been embroiled in conflict since 2013

Violence has quickly spread, with reports of mass killings nationwide

(CNN) Armed groups raided a South Sudanese school and seized 89 children who were taking their exams, the United Nations said Saturday.

The abduction occurred near Malakal, where thousands of people have taken refuge following months of violence in the nation.

Kidnappers gathered around a community and conducted house-to-house searches, according to the U.N. children's agency. It said the victims included boys over age 12, who were taken away by force.

U.N. officials warned the abductors that they're violating international law.

"The recruitment and use of children by armed forces destroys families and communities," said Jonathan Veitch, the UNICEF representative in South Sudan.

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