Juba - Heavy fighting broke out Wednesday in South Sudan's war-ravaged northeastern town of Malakal between rival factions of government troops, aid workers and local radio stations said.
An aid worker in the town, who asked not to be named, said they could hear the sound of large explosions beginning shortly after dawn.
Independent Radio Tamazuj spoke to citizens in Malakal, capital of the key oil producing Upper Nile state, who reported "gunfire from all sides."
Juba-based Eye Radio broadcast similar reports from the town, which was left in ruins last year after swapping hands repeatedly between rebels and government troops.
South Sudan's civil war started in December 2013 when President Salva Kiir accused Riek Machar, who had been sacked as vice president, of attempting a coup.
The two sides have been locked in sporadic and often intense fighting, leaving tens of thousands dead and over half of the country's 12 million people need aid, according to the UN.
Clashes on Wednesday were reportedly between rival factions in government forces, between troops loyal to the state governor and those of rogue general Johnson Olony, an ex-rebel who commands a pro-government ethnic Shilluk militia.
Olony was last month ordered to report to army headquarters after the United Nations he had abducted perhaps hundreds of child soldiers.
Neither the government nor the army could immediatly provide information on the fighting.
Peace talks in Ethiopia's capital Addis Ababa collapsed in March, and the impact of over 16 months of war continues to grow.
The number of civilians sheltering in UN peacekeeper camps continues to rise, with now over 117 000 civilians, with almost 5 000 more civilians fleeing into Malakal's already overcrowded camp earlier this month from fighting in Upper Nile.
Three aid workers transporting relief supplies from Malakal to Melut have not been heard of since they disappeared on 1 April, UN Ocha said.
Oxfam this week warned that "skyrocketing inflation, conflict and collapsed markets are pushing people in South Sudan to breaking point".
South Sudan's currency, the pound, now trades at half its official value on the black market, Oxfam said.
Food prices in key battleground northern and eastern states of Jonglei, Upper Nile and Unity have soared by 300 percent, according to the UN.