The United Nations says it has launched a “massive” humanitarian aid operation in South Sudan's Jonglei state, where tribal clashes killed an undetermined number of people and displaced tens of thousands.

The Office for the Coordination of Humanitarian Affairs says tens of thousands of people have returned to the town of Pibor now that the fighting has ended. But officials say an estimated 50,000 urgently need aid, including basics like food, water and medical care.

Spokeswoman Elizabeth Byrs told VOA on Friday the food distribution has already started, and that U.N. agencies are finalizing emergency plans for water, health care, shelter and sanitation.

The violence broke out in Pibor last week when about 6,000 men from the Lou Nuer tribe attacked areas controlled by the rival Murle tribe.

A local South Sudanese official said Friday the violence killed more than 3,000 villagers, including more than 2,000 women and children. U.N. officials have not confirmed those figures but say at least several dozen people have died.

Government forces have taken control of Pibor and the U.N. has vowed to increase its peacekeeping presence in the area.

Meanwhile, U.N. refugee agency chief Antonio Guterres is scheduled to visit South Sudan's capital, Juba on Saturday to discuss the humanitarian situation there.

The U.N. estimates more than 360,000 people have fled into South Sudan from neighboring Sudan in recent months.

Sudan's government is battling rebels in Blue Nile and Southern Kordofan states, which border South Sudan. The rebels are believed to support the south, which split from the north in July.

Guterres is also scheduled to visit Sudan on Tuesday to discuss humanitarian needs in that country.

The U.N. has asked Sudan to allow foreign aid groups to enter the war-torn states and reach people in need of assistance. Khartoum has denied the request.

The world body cites reports suggesting that food shortages and malnutrition rates have reached “alarming” levels in parts of Blue Nile and Southern Kordofan.