At least 20,000 people, mostly women and children, have fled into the bush to escape an outbreak of violence between rival tribes in South Sudan's Jonglei state, UN officials estimate.
The United Nations had moved combat troops to the remote town of Pibor to prevent an attack amid mounting inter-ethnic violence, originally triggered by cattle raids, between the Lou Nuer ethnic group and the Murle group.
Some 6,000 Lou Nuer men had gathered for an attack, and were spotted during UN air patrols.
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In violence over the past week, gunmen burned huts and looted two Doctors Without Borders outposts, the Guardian reported.
Lisa Grande, UN humanitarian coordinator for South Sudan, told the Guardian that "probably well over 20,000" people had fled into the bush.
In a statement, the group Save the Children said that up to 25,000 women and children have fled the fighting, and are in hiding.
The group said:
"They are likely to be terrified, and without clean water, shelter or enough food. Many could be injured. Children in the area already live in continual fear of violence and are often abducted in raids. If fighting continues, thousands more could be killed, maimed, abducted or recruited to fight."
The BBC said that "scores" of people have been killed in the most recent violence, on top of the estimated 1,000 people who have died in ethnic clashes in Jonglei state in recent months, according to the UN.
John Boloch of South Sudan's Peace and Reconciliation Commission told Sudan Catholic Radio News that at least 150 people had been killed in the past two days, the BBC said.
This is the latest round in a cycle of violence which has lasted several months — in one incident last year some 600 ethnic Lou Nuer were killed.
But South Sudan's Information Minister Barnaba Marial Benjamin told Agence France-Presse that "Pibor is under the full control of the government, and the Lou Nuer have been ordered to return to their homes, and they are starting to do so."
The violence threatens to destabilize South Sudan, the world's newest country, the Guardian said. South Sudan seceded from Sudan in July, after years of civil war that left more than 1.5 million people dead.
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