South Sudan's government says it is sending more troops and police to the town of Pibor, to deal with an outbreak of ethnic violence.
On Saturday, members of the Lou Nuer group attacked Pibor, home to the rival Murle group, in the latest of a series of reprisal attacks over cattle raids.
Tens of thousands of the Murle fled reports the BBC.
Some 6000 Lou Nuer fighters are chasing them, reportedly to take revenge for previous attacks and to rescue dozens of abducted children.
South Sudan's President Salva Kiir has called on them to stop their advance.
Charity Medecins Sans Frontieres says it is "extremely worried" after losing contact with some 130 staff in Pibor after they fled the violence.
A hospital and other parts of the town were set alight on Saturday.
The government said it was deploying more troops and an additional 2000 police to Pibor.