By Denis Dumo

JUBA (Reuters) - South Sudanese soldiers and rebels said they clashed in a state bordering Sudan, killing at least 15 people as violence raged on in the world's youngest nation after months of failed talks and peace accords.

The army said it moved in after spotting rebels loyal to former vice president Riek Machar trying to steal cattle in the small village of Adaab el Bahr in Unity State on Wednesday evening.

The rebels then caught them in an ambush, which carried on for just short of an hour, said the military's deputy spokesman Col. Santo Dominic Chol.

South Sudan seceded from Sudan in 2011 - a development greeted at the time with mass celebrations inside the landlocked, oil-producing state. Aid agencies and world powers promised support.

But fighting, largely along ethnic lines, erupted in 2013 after President Salva Kiir sacked his longtime political rival Machar.

A peace deal, agreed under intense international pressure and the threat of sanctions, brought Machar back to the capital Juba in April, but he fled after more clashes and the violence has continued.

Maj. Dickson Gatluak Jock, a spokesman for the Machar-allied SPLA-In-Opposition, said his forces killed 20 soldiers in Adaab el Bahr - higher than the government estimate of 15 - and another 23 in a fight in Tore in Central Equatoria state on the same day.

Another rebel spokesman, Col. William Gatjiath Deng, said they had also seized control of three towns in fighting in the two states on Tuesday and Wednesday. The army denied that report.

(Editing by George Obulutsa and Andrew Heavens)