Rebels with the South Sudan People's Liberation Movement in Opposition (SPLM-IO), allied with rebel leader Riek Machar, on Friday attacked Raja town, the capital of Lol state.
SPLM-IO Secretary-General Tingo Peter confirmed his forces now controlled Raja after clashing with government troops.
"Since 12 o'clock, we captured Raja, and it is now totally under our control. Even now, our forces are looking for the governor. They are trying to see where he is," Peter said.
Lol Governor Rizik Zachariah Gassan and his entire cabinet fled the area, according to Peter.
Peter said the SPLM-IO was asking civilians in Raja to remain calm as their forces combed the town for government soldiers who might be hiding in residential areas.
"Now they are with us and they are safe," Peter said. "We give them protection, but the problem is we can't manage to offer them food or water."
He urged the international community to intervene and help civilians in Raja town. He said the SPLM-IO would also protect all aid workers in the area.
Raja resident Issa Ramadan confirmed to South Sudan in Focus that Raja was under the control of SPLM-IO rebels. Ramadan said he could see from inside his home that rebel forces were moving around the town with tanks they captured from government forces a few hours earlier.
"At around 2 p.m., the rebels attacked the town and now they took over," Ramadan said. "Many citizens have run out into the bush and some have sought refuge in the Catholic church compound."
The coordinator of Lol state in Juba, Ibrahim Surur, confirmed that there was fighting in his state but declined to provide details. He said he tried to call officials in Raja, but most were not answering their calls.
Meanwhile, the World Food Program (WFP) said Friday that three of its contract workers were killed in Wau this week.
Daniel James, Ecsa Tearp and Ali Elario, all citizens of South Sudan, reportedly died Monday as they tried to make their way to a WFP warehouse, where they worked as porters.
A WFP statement released Friday said two died of machete wounds and the third was shot to death.
"We are outraged and heartbroken by the deaths of our colleagues, who worked every day to help provide lifesaving food to millions of their fellow countrymen," said WFP Country Director Joyce Luma. "Their dedication will not be forgotten, and we call on the South Sudanese authorities to hold those responsible for this unspeakable violence accountable for their actions."
WFP learned of the workers' deaths Thursday from the subcontractor that employed them.