Major General Lul Ruai Koang told AFP that the fighting erupted on Saturday as security forces carried out an operation to disarm civilians in the area which has seen deadly inter-communal clashes.

More than six years after a civil war broke out in the country, and in the absence of a functioning government, many communities are flush with weapons, which they keep for protection or defence against cattle raids.

The violence in Tonj began after several armed youths got into a disagreement with soldiers. An initial armed confrontation was brought under control, but according to Koang the youths mobilised others for an attack on the army position.

"On the latest, the number of those killed, I can confirm to you that it rose to 127," Koang said, adding that 45 of those killed were security forces and 82 were youths from the area.

A further 32 soldiers were injured.

Koang said two military officers involved in "triggering the clashes" had been arrested, and that the situation in Tonj had calmed down.

South Sudan is emerging from a six-year civil war that left 380,000 dead and millions displaced, and disarmament is a major stumbling block.

Experts have warned against operations that coerce people to lay down their guns without proper planning, as some communities could find themselves unable to protect themselves after their weapons are removed.

"The clashes should be an opportunity to rethink the approach to disarmament. What is the point of removing guns without addressing what drives folks to arms themselves?" Geoffrey Duke, head of the South Sudan Action Network on Small Arms, said on Twitter.

"We can take guns away this week & they buy a new one next week (as) long as they still see the need to have (one)."