South Sudan's army has clashed with the "White Army" ethnic militia, a spokesman has said, accusing rebels of mobilising the force despite the offer of a truce to end the conflict in the country.
The White Army - made up largely of Nuer youths who dust their bodies with ash to protect themselves against insects - clashed with government troops near the town of Bor five days after rebels were driven out, army spokesman Philip Aguer said on Sunday.
The army used helicopter gunships against the militia advance near Bor, Aguer said.
"They then dispersed and returned back," he added.
The fighting which started earlier this month is pitting mainly Nuer rebels against military forces loyal to President Salva Kiir of the Dinka tribe.
South Sudan's minister of information told the Associated Press earlier on Sunday that most of the militia had disbanded and returned home.
Michael Makuei Lueth said Nuer community leaders in Jonglei state persuaded the White Army fighters to go home. Lueth said only a "very few" refused to disband. He said an exact number was not known.
Clashes that first erupted around the capital Juba on December 15 have quickly spread, with the rebels backing sacked Vice President Riek Machar, a Nuer.
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Fighting has left at least 1,000 dead and split the east African country barely two years after it won independence from Sudan.
It has also raised fears of an all-out civil war between the Dinka and Nuer which could destabilise the fragile region.
Machar made no immediate comment on the rebel force or on the government's offer of a ceasefire on Friday.
Witnesses spoke of panicked civilians fleeing Bor to escape another round of bloodletting. Bor was retaken by government troops after several days of heavy fighting.
The army said rebels were also mobilising youths and armed civilians for another attack on Malakal, the capital of the oil-producing Upper Nile state. Rebels were pushed out of the town on Friday.
As well as offering a truce, Kiir's government said it would release eight of 11 senior politicians, widely seen to be Machar allies, arrested over an alleged coup plot against Kiir.