South Sudan says it will put key leaders of an alleged coup it claims triggered weeks of fighting on trial, in a move likely to threaten a fragile ceasefire with rebels.

The announcement, made late Tuesday by Justice Minister Paulino Wanawila, underscored the animosity that still reigns between government forces under President Salva Kiir and a loose coalition of rebel fighters seen as loyal to Kiir's sacked deputy, Riek Machar.

Both sides implemented a ceasefire last Friday, but combat has only eased, not ended.

READ: South Sudan warring parties finally call a truce

Kiir accused Machar and other former officials of fomenting a coup against his government after fighting broke out in the capital Juba on December 15.

Eleven ex-officials were arrested, while Machar -- who denied any coup plot -- fled. Fighting quickly spread across the country. Aid groups say up to 10,000 people have been killed in the conflict.

The release of the prisoners has been a key demand of the rebels.

Prisoners have been divided into three categories: some to be tried, others suspected of involvement but who will be released to a third country -- not yet named -- and others who would be released without charge.

Wanawila late Tuesday repeated the coup allegation against Machar and said some rebel leaders would be tried.

"If someone violates the law you don't go and torture that person, you prosecute that person according to the law," he said.

He said six men were targeted for trial.

Four were in detention: Pagan Amum, former secretary general of the ruling party, ex-national security minister Oyai Deng Ajak, former ambassador to the US Ezekiel Lol Gatkuoth, and ex-deputy defence minister Majak D'Agoot.

Two others were on the run: chief rebel delegate Taban Deng who signed the ceasefire, as well as Alfred Ladu Gore, an experienced guerrilla fighter and respected leader especially in his home region surrounding the capital Juba.

Kiir has hinted he could use his power to grant amnesties but has said that legal processes must be completed first.

READ: Kiir: Conflict will be resolved via talks

Both sides say the other has already broken the ceasefire, but insist they are committed to the deal.

Government and opposition rebels are still fighting for control in key areas, with the United Nations calling the situation "fragile".