Nuer and Dinka people have a long history of rivalry
At least 140 people have been killed in ethnic clashes in Southern Sudan, officials say, as aid agencies warn that the country faces a return to war.
Deputy governor of Warrap state Sabino Makana said members of the Nuer group attacked Dinka cattle herders and seized thousands of animals.
Most of the violence happened over the weekend in very remote areas, he said.
The UN says more than 2,000 people have been killed in ethnic violence in the south since January 2009.
More people died in Southern Sudan than in Darfur last year, however a minister has denied that war could resume.
The BBC's Peter Martell in the southern capital Juba says the Nuer and Dinka people have a long history of both rivalry and co-operation.
By Peter Martell
BBC News, Juba
Violence of this scale and intensity is creating a lot of concern in Juba that things are out of control in the south's more remote regions.
Government ministers say they are doing all they can - they have launched disarmament drives and, with the election coming up, they are trying to tell people not to use guns to solve their disputes.
But that is an extremely difficult message to get out to heavily armed groups of people in remote areas who are angry over long-lasting enmities.
He says 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 destabilise 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.
"This will never happen because all parties - [President Bashir's] National Congress Party and SPLM are keen not to go to war again," he told the AP news agency.
He did admit, however that there was a "humanitarian catastrophe" in the south, which remains one of the poorest areas of the world, leading to frequent clashes over resources.