* Ceasefire in place but some clashes still reported
* Peace talks to resume in Addis Ababa around Feb. 7
* Conflict brought South Sudan to brink of civil war
By Aaron Maasho and Edmund Blair
ADDIS ABABA, Jan 30 (Reuters) - The United States called on South Sudan on Thursday 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 and 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 closely watched by regional and Western powers trying to avert a collapse into civil war.
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 SPLM, Pagan Amum.
"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," U.S. Deputy Secretary of State William Burns said on the sidelines of an African Union summit in Addis Ababa.
He added 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 going on.
"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 told Reuters, also speaking in the Ethiopian capital.
A source in the Kenyan presidency said Kiir might be holding on to the four to strengthen his hand in negotiations, due to resume on Feb. 7. "If he plays all his cards, then what?" he said.
South Sudan's current Justice Minister Paulino Wanawilla Unago said on Tuesday Machar and others could still face treason charges - a prosecution that could still disrupt peace efforts.
The clashes erupted after months of political tension between backers of Kiir, who is an ethnic Dinka, and rivals who supported Machar, a Nuer. Rights groups have accused both sides of atrocities.
The United States 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. (Edited by Drazen Jorgic and Andrew Heavens)