Lul Ruai Koang told Reuters the two-day clashes erupted over the weekend after some armed youths in the Greater Tonj area started engaging the security forces.
It was not immediately clear what sparked the fight, he said, adding that an investigation has been started.
“Among the dead are 55 members of the security forces and 26 civilians. Another 31 servicemen were also wounded,” Koang said, adding that the numbers could rise.
The wounded security personnel were flown to the military hospital in the capital Juba for treatment, he said, adding that calm had returned to the area as authorities restored order.
The national government, formed this year after a deal to end a conflict that broke out in 2013, launched a disarmament exercise in Tonj county last month, saying that armed militias in the area were driving inter-communal violence.