Government officials in South Sudan say new tribal clashes in troubled Jonglei state have killed 22 people.

Authorities said Monday that members of the Murle tribe attacked and burned three villages belonging to the rival Lou Nuer tribe.

The attack appeared to be in retaliation for a deadly raid by the Lou Nuer tribe on Murle-controlled areas in late December, including the town of Pibor.

The United Nations said peacekeepers in the region found dozens of bodies following the December attack. One local official said more than 3,000 people were killed, but that number has not been confirmed.

The fighting forced tens of thousands to flee. The U.N. says while most have returned home, an estimated 50,000 people need basic aid such as food, water and medical care.

The two tribes fight frequently over land and cattle.

The United Nations says it is worried about tension 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.