The violence erupted Thursday in Jonglei state after a cattle raid by tribesman in the country's east.
South Sudan became independent from Sudan on July 5 to much international fanfare, but diplomats have expressed mounting concerns about stability in the country and tensions with the Khartoum government.
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A UN team has been sent to Jonglei and the head of the UN mission to South Sudan, Hilde Johnson, has appealed for restraint.and the UN mission to South Sudan reported the death toll, said UN deputy spokesman Farhan Haq.
Jonglei Governor Kuol Manyang Juuk said women and children were among the dead and that the fighting started in Bier sub-county.
He said Murle tribe members were suspected of attacking five villages of the Lou Nuer tribe in Bier. He said cattle had been stolen and some women and children abducted.
Justice Minister John Luk Jok said he saw bodies strewn across the scene of the raid.
The two tribes frequently clash over land and cattle.
South Sudan became the world's newest nation in July after voting to separate from the Arab north.
Decades of fighting with the north left South Sudan as one of the poorest and least developed places in the world, with conflict in nine of its 10 states.
