BBC News with Iain Purdon

The United Nations mission in South Sudan says government troops have retaken control of the centre of the town of Pibor, which was captured by tribesmen on Saturday. But a spokeswoman for the mission, Liz Grande, told the BBC the situation was still fluid. She said thousands of Lou Nuer tribesmen were still on the peripheryof the town, where they were endangering civilians who'd fled the fighting. Here's our East Africa correspondent Will Ross.

The next few days will show to what extent the government of South Sudan is in control of this fledgling nation. President Salva Kiir has called on the 6,000 armed men from the Lou Nuer tribe to halt their attacks on the rival Murle people. But they appear determined to take revenge for past cattle raids and to rescue dozens of abducted children. The United Nations peacekeeping force has a robust mandate, but it's not clear whether it has the capacity to stop the violence, which is now on an alarming scale.