A UN official confirmed the deaths, saying the clashes occurred some time since the beginning of the year.
Rise in violence
But reports of the clash surfaced only after a UN security team visited the area this week, the AFP news agency reported.
"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," Lise Grande, the UN deputy resident and humanitarian coordinator in south Sudan, told AFP.
"This is a matter of deep concern."
A sharp rise in tribal violence in recent months has killed more than 2,000 people, including many women and children, and displaced another 250,000 people, the United Nations has said.
Rival tribes from Sudan's south have clashed for years in disputes often caused by cattle rustling and long-running feuds, but violence has soared this year.
The Sudanese People's Liberation Movement (SPLM) has blamed at least some of the inter-tribal fighting on interference by Khartoum, which they say is arming civilians and militias to cause unrest ahead of elections and a 2011 referendum on southern independence.
Others have said the blame should be partly put on rivalry between southern leaders, complicated by the 20-year war that often pitched southern ethnic groups against each other.