Unknown gunmen opened fire outside a south Sudanese army base, raising tensions days after around 40 soldiers and civilians were killed by tribal fighters, officers said on Tuesday.
The confrontation came after a string of ethnic clashes in Sudan's semi-autonomous south which have threatened a fragile peace, achieved in a 2005 accord that ended two decades of north-south civil war.
Tribal fighters killed seven south Sudanese soldiers and around 30 civilians escorting a convoy of river barges carrying UN food aid on Friday, the south's Sudan People's Liberation Army (SPLA) told Reuters.
The United Nations said the attack on the Sobat river was launched by members of the Jikany Nuer group who suspected the boats were taking food and arms to their Lou Nuer enemies in the settlement of Akobo.
On Monday morning, another group fired guns, apparently into the air, outside a southern army base in Nasir, in Upper Nile State, close to the scene of the river attack, said SPLA spokesman Malaak Ayuen Ajok.
"No one responded to the provocation," said Ajok. "The SPLA soldiers are keeping in their bases and there was no fighting. No one was injured. The situation is now calm."
Ajok said locals in Nasir were angry after hearing false claims the southern army was sending food and support to their Lou Nuer rivals. Senior SPLA officers were now visiting the area to try to repair relations, he added.
UN sources, speaking on condition of anonymity, said there were worrying signs Lou Nuer fighters were arming for a possible revenge attack.
South Sudan's ethnic groups have long clashed over disputes, most of them related to cattle rustling. The fighting has peaked in recent years amid widespread political discontent and a flood of weapons, left over from the north-south civil war.
The 2005 peace accord promised national elections, due in February, and a referendum on southern secession in 2011. But many in the south are frustrated by the continuing lack of development and stagnation in the region's oil-based economy.