The South Sudanese government on Monday declared army chief of staff Paul Malong a rebel and accused him of being behind a series of attacks last week.
The General, who had led President Salva Kiir's campaign against rebels, has been under house arrest since May.
Kiir sacked him following a string of military resignations by senior generals alleging abuses and ethnic bias.
Malong initially fled the capital Juba for his home state of Aweil following his dismissal, raising the possibility he might join opposition forces, before returning to Juba.
Malong loyalists started joining rebels and in November, Kiir released him to exile in
Kenya.
South Sudan, which became the world's newest country after splitting from Sudan in 2011, plunged into war in late 2013 after Kiir sacked his deputy Riek Machar.
The dispute erupted into fighting that spread across the country, largely along ethnic lines between forces loyal to Kiir and Machar.
Kiir's spokesman Ateny Wek said Malong was ordering his commanders in South Sudan to attack the government. He cited an audio recording obtained by its intelligence services.
"Malong is a former chief of staff of the army but in accordance with the tape, he's a rebel. The government and the security committee will come with an appropriate response," he told a news conference.
Lucy Ayak, Malong's wife, said the accusations were baseless.
"This audio is a fake recording," she told Reuters by phone from Nairobi. "Every time the government accuses him of wanting to launch an attack, which is not true."
Last week, clashes broke out near Juba between government troops and rebels, the latest violation of a ceasefire signed in December.
The deal reached in the Ethiopian capital Addis Ababa, between Kiir's government and a myriad of opposition groups, had aimed to end the four-year-old war in which tens of thousands of people have been killed.