An armed group in South Sudan has abducted 89 boys in a northern state, the UN children's agency has said.

The boys, some as young as 13, were abducted near Malakal, the capital of Upper Nile state, according to a UNICEF statement on Saturday.

The children were abducted while doing their exams in the town of Wau Shilluk, said the statement, citing a UNICEF education team.

Armed men surrounded the community and searched house by house, forcibly taking boys older than 12, said witnesses, according to the UNICEF statement.

"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," said Jonathan Veitch, the UNICEF representative in South Sudan.

It is not clear which group carried out the abductions. Watchdog groups have persistently accused South Sudan's warring factions of actively recruiting and using child soldiers.

South Sudan's government denied recent allegations by Human Rights Watch that its military is targeting children for enlistment.

Source: Agencies