At least 30 South Sudanese soldiers were killed and 13 injured in clashes between forces loyal to Vice President Riek Machar and a splinter group, Machar's party spokesman said in a statement on Saturday.
In a move that risks derailing the country's already fragile peace process, rival military leaders of Machar's party SPLM/A-IO, announced on Wednesday that the vice president was ousted as head of his party and its armed forces.
As a result, the party's chief of staff, First Lieutenant General Simon Gatwech Dual, was nominated interim party leader from the military wing.
On Thursday Machar, who played a leading role in brokering a peace deal in 2018 with President Salva Kiir, accused the rival military leaders of trying to block the country's peace process.
The latest clashes occurred in Magenis, in the country's Upper Nile region, between forces loyal to General Dual and those backing Machar.
Machar's party's spokesman Lam Paul Gabriel said the party's forces responded "in self defence" and killed two major Generals and over 27 soldiers.
He said those fighting on the side of the SPLM/A-IO lost 3 soldiers during the attack.
Reuters could not independently verify the report of killings. The military wing was not immediately available for comment and communication networks are patchy in the Upper Nile region.
The other side denied having suffered heavy losses and having launched an offensive.
"We have so far lost one soldier and two others sustained injuries. The fighting is ongoing," Chuol Deng, deputy spokesman for General Dual told Radio Tamazuj, a south Sudanese radio station earlier on Saturday.
South Sudan gained independence from Sudan in 2011 but descended into fighting two years later when forces loyal to Kiir and Machar clashed in the capital.
That sparked the massacre of hundreds of civilians in Juba from Machar's Nuer ethnic group and a spiral of brutal ethnic violence and revenge killings.
The civil war killed 400,000 people and triggered Africa's biggest refugee crisis since the 1994 Rwandan genocide.