Fighting between government forces, rebels and rival tribes has cut off 100,000 people from urgently needed food and medical aid in South Sudan's east, U.N. and aid officials said on Wednesday.
South Sudan's army is facing a rebellion from local politician David Yau Yau in the vast Jonglei state, and new clashes have broken out between the rival Lou Nuer and Murle tribes.
Western powers are worried the violence will escalate into full civil war, undermining stability in the young African country, where weapons are plentiful after decades of conflict with Khartoum that led to its secession from Sudan in 2011.
U.N. humanitarian aid chief Valerie Amos said the new fighting made it impossible to supply some 100,000 people in Pibor county in Jonglei State with "life-saving assistance".
"The fighting is threatening the lives of ordinary people," Amos said in a statement. The United Nations did not have enough helicopters to deliver aid to Jonglei where overland travel is impossible in the rainy reason, she said.
Medicins sans Frontiers (MSF), one of the few aid groups operating in Jonglei, said more than 120,000 people had been forced to flee clashes between the army and Yau Yau rebels.
A United Nations source said armed Lou Nuer youth had attacked several Murle villages in the past two weeks. Fighters loyal to Yau Yau, who is popular with his Murle tribe, had come to help fight back.
Separate tribal clashes were also reported in Unity state, site of several oilfields. In one incident, attackers apparently burnt a hut in a village with a woman and three children inside, said a U.N. source, asking not to be named.
The United Nations has not published any casualty figures of the Jonglei fighting despite a large presence of peacekeepers. Critics say the world body does not want to embarrass the government.
South Sudan accuses Khartoum of supplying Yau Yau with weapons. Diplomats say the claims are credible but South Sudan's army is also fuelling dissent with abuses such as rape, killings and torture committed during a state disarmament campaign.
Last week, the United States, South Sudan's biggest ally, said Juba was not doing enough to protect civilians and urged the army to stop attacking U.N. staff and looting aid agencies.
South Sudan has struggled to turn its army, a loose group of former guerrillas formed during the civil war, into a professional force.
Tribal violence has killed more than 1,600 people in Jonglei since South Sudan's secession, hampering plans to explore for oil with the help of France's Total and U.S. firm Exxon.