Human Rights Watch says South Sudan's military is actively recruiting boys into the armed forces despite assurances to the contrary.

The rights watchdog said in a statement Monday that boys as young as 13 have been recruited as soldiers, often by force, in South Sudan's oil-producing state of Upper Nile.

Kuol Manyang Juuk, South Sudan's defense minister, denied the allegations, saying standing orders forbid the army from recruiting children.

South Sudan's warring factions have faced persistent allegations of using child soldiers in a conflict that pits government troops against rebel forces loyal to a former vice president.

Human Rights Watch said it had spoken to about a dozen children or young men who were under 18 when they fought in 2014.