By Atem Simon
Clashes broke out Friday between South Sudan's government troops and forces loyal to sacked vice president Riek Machar in the rebels-held Bor, the capital of oil-rich Jonglei State, military spokesman Colonel Philip Aguer told Anadolu Agency.
He declined to give details about casualties.
There was no comment from Marchar's forces.
On Thursday, South Sudanese army chief-of-staff Gen. James Hoth Mai said that army troops were advancing on Bor, which was recaptured two days ago by Machar's forces.
"We will be there soon," he told a press conference.
Mai said that his troops were also advancing on Unity State capital Bentiu.
South Sudan has been shaken by violence since mid-December, when President Salva Kiir accused Machar of standing behind a failed coup attempt.
A subsequent government crackdown landed many of Machar's supporters in jail, but the former VP managed to flee to Jonglei State, a stronghold of his Nuer tribe.
Although Machar had initially denied involvement in the alleged coup, his forces have reportedly captured parts of both the Jonglei and Unity states.
On Wednesday, the United Nations expressed deep concern over what it described as mounting evidence of grave human rights violations and the extra-judicial killing of civilians and captured soldiers in strife-torn South Sudan.