More people died in Southern Sudan than in Darfur last year, however a minister has denied that war could resume.
The Nuer and Dinka people have a long history of both rivalry and co-operation. The latest clashes came to light only after a UN security team flew over the area around the city of Tonj two days ago. Senior UN official Lise Grande said it was a "matter of deep concern".
"Local sources on the ground said that at least 140 people had been killed, 90 wounded and 30,000 head of cattle had been stolen," she said. The Warrap state deputy governor told Reuters news agency that the Nuer attackers killed 139 Dinka herders and wounded dozens more on Saturday. He said many of the Nuers also died, but he did not have reliable figures.
The North and South fought a 22-year civil war that left some 1.5 million people dead. A 2005 peace deal ended the conflict and created a power-sharing government. But 10 international aid agencies warned on Thursday that a "lethal cocktail" of rising violence, chronic poverty and political tensions was pushing the accord towards collapse.
Southern politicians accuse President Omar al-Bashir's allies of arming rival groups in the south to stoke up trouble.
They say Mr Bashir wants to destabilize the region to sabotage a national election planned for April, and a referendum on Southern independence the following year. But Humanitarian Affairs Minister Abdul Bagi Gilani said there was no possibility of a return to war.