Officials in South Sudan say dozens of people have been killed in tribal clashes over cattle in Jonglei state.

State governor Kuol Manyang Juuk says fighters from the Murle tribe attacked several villages of the Lou-Nuer tribe on Thursday. The two tribes fight frequently over land and cattle.

South Sudanese and U.N. officials say at least 40 people were killed but some reports put the death toll much higher.

South Sudan became the world's newest independent country last month, after voting to split from the north.

The new country has been plagued with tribal violence and clashes between the government and rebel militias.

The government has urged South Sudanese to work together, saying the country has the potential to become the breadbasket of eastern Africa.