(CNN) -- A huge aid effort is under way in a remote area of South Sudan to help an estimated 60,000 people who fled their homes to escape roaming fighters, the United Nations said.
Some 6,000 armed men from the Lou Nuer tribe marched on an area of Jonglei state, which is home to the rival Murle tribe, attacking the town of Pibor last weekend.
Although the Lou Nuer fighters have left, following negotiations with U.N. peacekeepers and the South Sudan authorities, help is urgently needed for those who fled, the U.N. Mission in South Sudan (UNMISS) said Friday.
The special representative for the U.N. Secretary-General in South Sudan, Hilda F. Johnson, is traveling to Pibor on Saturday to see the situation, UNMISS spokesman Kouider Zerrouk said.
The South Sudan government has declared Jonglei a "humanitarian disaster area" and appealed for international help.
Aid workers estimate that 60,000 people are in need of assistance and the U.N. World Food Programme voiced concern Friday that food shortages in the area "could reach crisis levels." It has already made emergency food deliveries.
U.N. Humanitarian Coordinator Lise Grande said some of the people who fled into the bush to escape the fighters are starting to return to their homes, according to a statement.
U.N planes are also trying to find other families still hiding out in the bush, she said.
"But there are a number of villages that were burned completely to the ground, for example, Likuangole, and in that case people are not coming back and that's because there is nothing to come back to," she said.
Ethnic tensions in Jonglei state have flared as tribes fight over grazing lands and water rights, leading to cattle raids and abduction of women and children.
Government officials have urged the two ethnic groups to return women and children abducted in the spate of violence.
The violence in Jonglei state is the latest to rock South Sudan, which officially gained its statehood in July after separating from neighboring Sudan to the north.
U.N. High Commissioner for Refugees Antonio Guterres is also in South Sudan this weekend, to see other areas affected by conflict.
Among them will be a refugee site in Mabaan, where tens of thousands of refugees have sought help since fleeing fighting across the border in Sudan's Southern Kordofan and Blue Nile States.
U.N. flights have delivered thousands of tents, kitchen sets, blankets, jerry cans, plastic sheets, sleeping mats, mosquito nets and other essential items to Mabaan and another refugee camp at Malakal in the past two-and-a-half weeks.
Decades of civil war between the north and south, costing as many as 2 million lives, ended with a U.S.-brokered peace treaty in 2005.
But before South Sudan gained independence in July, human rights monitors expressed concerns that long-standing grievances could end in violence consuming the region again.
The United Nations estimates that more than 1,100 people died and 63,000 were displaced last year by inter-communal violence in Jonglei state, not taking into account the latest clashes.
U.S. President Barack Obama gave his approval Friday for the sale of weapons and defense services to South Sudan.
The decision could open the door to South Sudan acquiring air defenses. It has accused Sudan of carrying out aerial bombardments on its territory.
The White House condemned air raids by the Sudan Armed Forces on South Sudan in November, saying: "These provocative aerial bombardments greatly increase the potential for direct confrontation between Sudan and South Sudan.
The memorandum sent by Obama Friday to U.S. Secretary of State Hillary Clinton said the "furnishing of defense articles and defense services to the Republic of South Sudan will strengthen the security of the United States and promote world peace."
CNN's Laura Smith-Spark and Pierre Meilhan contributed to this report.