Tens of thousands of South Sudanese people are fleeing from their homes, after inter-ethnic clashes around the town of Pibor.
The United Nations is warning villagers to run for their lives ahead of advancing fighters from a rival tribe.
Parthrsary Rajendran, head of mission for Medecins Sans Frontieres in South Sudan, speaks on the phone from the capital Juba.
Dr Sara Pantuliano, Sudan analyst at the Overseas Development Institute, says that this is part of a "long-standing conflict" in the region.
Conflict is a "fact of life" between these two social groups, she adds, but the cattle wars have now become more like "military assaults" as the authority of the elders and chiefs has diminished as a result of the "massive proliferation" of weapons in the region.
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