South Sudan and the United Nations have increased their troop presence in troubled Jonglei state in a bid to contain tribal clashes in the area.

A group of 6,000 armed men from the Lou-Nuer tribe has attacked the towns of Pibor and Lukangol during the past week, setting fire to huts and sending thousands of civilians fleeing.

Pibor is home to the Murle tribe, which was blamed for attacks on Lou-Nuer areas in August.

The two tribes fight frequently over land and cattle.

The United Nations says it is worried about the ethnic tensions in Jonglei, one of the worst-hit states for ethnic violence in South Sudan. The U.N. estimates more than 1,000 people were killed in ethnic clashes in the country last year.