South Sudan gained independence from Sudan in 2011. Sovereignty was assured in a 2005 peace deal ending civil war, though ethnic fighting, rebel concerns and border disputes have threatened the fragile peace.
Gabriel Jok Riak, commander of the Sudan People's Liberation Army, told the independent Sudan Tribune his forces in the border state of Unity observed "unusual movement" of Sudanese forces beginning last week.
The commander told the newspaper his forces were hit with heavy gunfire from Sudanese soldiers.
"The SPLA responded in self-defense to firing from the Sudan Armed Forces, which crossed into areas beyond [the border]," he said in an interview published Monday.
He said there were casualties reported during the melee but blamed Khartoum for undermining efforts to bring stability to the area.
"Sudan does not want peace to prevail in this region," he said.
There was no corresponding statement from Khartoum. The Sudanese government has expressed frustration with rebel activity in South Sudan in the past.
Both sides agreed to establish a demilitarized zone along the border and the Sudanese newspaper reports the area has been relatively quiet since last year.
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