The South Sudanese government has said an audio recording which reportedly revealed that former army chief of staff General Paul Malong Awan had turned rebel is authentic.
On Monday, President Salva Kiir accused his former ally of forming a rebellion and ordering attacks against his government.
Ateny Wek Ateny, the president's spokesperson, said the leaked tape revealed Gen Malong commanding his loyalists to attack the capital Juba and his hometown in Aweil.
"He [Malong] said he will not join Riek Machar but if Machar gives him forces then he will use those forces against the government in Juba," Mr Ateny told local media.
The government has accused Gen Malong of being behind a series of attacks last week, in violation of a ceasefire agreement signed between Juba and a number of rebel groups.
Gen Malong was sacked in May and placed under house arrest for fear that he would form a rebellion against President Kiir. He was released in November following mediation by Dinka elders but prevented from going to Aweil in the former Bahr-el-Ghazal State.
Since his appointment as army chief in 2014, Gen Malong had grown to enjoy more influence in the military than even the president.
Several Malong allies in the army resigned, joined or formed their own rebel outfits after his sacking and consequent house arrest.
Mr Kiir is keen to ensure that there is no split among the Dinka, who are still in a state of war with the Nuer, the second largest tribe, to which his nemesis Dr Machar belongs.
Gen Malong has however denied the accusations.
"I would want to make it clear to the people of South Sudan that the tape is fake and doesn't bear the slightest authenticity in it. The tape record is a clear imitation of my voice minted by my well-known enemies," Gen Malong is quoted by local media.
South Sudan plunged into war in December 2013 after President Kiir sacked his deputy Dr Machar.
According to the International Crisis Group, more than 100,000 people have lost their lives in the brutal four-year civil war that is fought largely along tribal lines.
Over two million others have become refugees, while one million South Sudanese remain internally displaced, according to the UN.