South Sudan's military spokesman says a rebel force has overrun part of a town in the country's east near the border with Ethiopia.
Col. Philip Aguer on Wednesday confirmed the fall of part of the town of Boma to rebels led by David Yau Yau.
The rebels claim they killed 50 soldiers, but Aguer denied that was true. He said he did not have casualty figures, however.
South Sudan accuses Sudan of supporting Yau Yau's rebellion, a strategy meant to block Juba's plans to build an oil pipeline into Ethiopia. South Sudan currently can only export oil through Sudanese pipelines.
Khartoum has repeatedly denied having ties to Yau Yau's rebels. A Swiss based research group, Small Arms Survey, said recently that Sudan has supplied weapons and ammunition to the rebels.