Officials in South Sudan say 57 people have been killed in renewed fighting between two rival tribes.

The officials said Friday the clashes broke out Wednesday when armed members of the Murle tribe attacked villages of the Lou Nuer tribe in Jonglei state.

About 6,000 men from the Lou Nuer attacked the town of Pibor and other areas controlled by the Murle last month.

One South Sudanese official said those attacks killed more than 3,000 villagers, including more than 2,000 women and children. U.N. officials were not able to confirm that figure but said at least several dozen people were killed.

Last week, the United Nations launched a relief operation in Jonglei to help an estimated 50,000 people displaced from their homes.

South Sudan is the world's newest country but has been wracked with tribal violence and fighting between the government and rebel groups almost from the start.

The country is also trying to cope with an influx of refugees from neighboring Sudan and a dispute over oil revenue with the Khartoum government.