The United Nations Children's Fund (UNICEF) says that an armed group in South Sudan has abducted 89 boys in a northern state.

UNICEF said in a statement on Saturday that the boys, some as young as 13, were abducted near Malakal, the capital of Upper Nile state.

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

Citing witnesses, the statement added that armed men surrounded the community and searched house by house, forcibly taking boys older than 12.

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.