The United States has called on South Sudan to release the last four of a group of detainees held on suspicion of trying to stage a coup, a move that could clear a major sticking point in talks to end weeks of fighting.
The senior political figures were arrested after clashes broke out between rival groups of soldiers in South Sudan's capital Juba in mid-December.
The violence quickly spread across the oil-producing nation along ethnic lines, killing thousands.
Groups fighting the government deny there was ever any plot to take power and have demanded the men's release during peace talks in Ethiopia, which are being closely watched by regional and Western powers trying to avert a collapse into civil war.
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South Sudan freed seven of the detainees, including the country's former justice and finance ministers, on Wednesday and handed them over to neighbouring Kenya.
But it said it was still examining evidence against the last four, including the former secretary general of the ruling The Sudan Peoples' Liberation Movement.
"We welcome the release of the seven detainees and we believe that is a positive step ... We will continue to urge the release of the remaining four detainees," US Deputy Secretary of State William Burns told the Reuters news agency on Thursday on the sidelines of an African Union summit in Addis Ababa.
He said he was concerned about fighting that had continued in the impoverished nation, despite a ceasefire agreed by both sides at the negotiations last week.
South Sudan President Salva Kiir accused his former vice president Riek Machar of starting the fighting in Juba in a bid to seize power.
Machar, who is in hiding, dismissed the charge and accused Kiir of acting like a dictator and using the fighting as an excuse to round up political rivals.
Fighting between troops loyal to both men spread to oil-producing areas before the ceasefire. Forces fighting the government come from disparate militias, and experts say Machar may not be able to order all of them to stop fighting.
South Sudan's Foreign Minister Barnaba Marial Benjamin said the investigation of the remaining four was ongoing.
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"As soon as it [the investigation] is over, the report again will go to the president and he has the option also of using his constitutional authority to grant a pardon or whatever," he said, also speaking in the Ethiopian capital.
A source in the Kenyan presidency told Reuters that Kiir might be holding on to the four to strengthen his hand in negotiations, due to resume on February 7.
"If he plays all his cards, then what?" he said.
South Sudan's current Justice Minister Paulino Wanawilla Unago said on Tuesday that Machar and others could still face treason charges - a prosecution that could still disrupt peace efforts.
The US is a major donor to South Sudan, which has almost no other source of government income outside oil revenues.
Washington, along with other powers, pushed negotiations that ended decades of civil war between southern fighters and Sudan's government in the north and led to the secession of South Sudan in 2011.