Thousands of young South Sudanese fighters loyal to ex-vice president Riek Machar have peeled away from an advance on a key town, the government says, but there was no immediate confirmation from Machar's supporters.
As the UN voiced concern about the youths' mobilisation, government spokesman Michael Makuei told AFP on Sunday that "most of them have returned home".
"According to our sources, local Lou and Dau Nuer chiefs have convinced the youths," he said. "So it looks like things are calming down."
The United Nations earlier on Sunday said it was "extremely concerned" over claims that thousands of armed youths allied to Machar were readying to attack in South Sudan, amid international efforts to broker a ceasefire.
Juba has claimed that Machar, the de facto leader of rebels fighting the government, has recruited up to 25,000 young fighters from his Nuer tribe in the eastern state of Jonglei and that they are "ready to attack any time".
According to Jonglei's acting governor Ogato Chan, the Nuer fighters were around 110 kilometres (70 miles) from the state capital of Bor, currently under the control of government forces.
"The situation in the town [Bor] is calm," he said. "The information is that they want to come and attack Bor but I am sure they will not attempt to do it because the SPLA forces will repulse them."
Rebel spokesman Moses Ruai Lat has said Machar was "not mobilising his tribe", the second-biggest ethnic group in South Sudan. He described the men instead as regular soldiers who had rejected the government and were not specifically drafted by Machar.
A spokesman for the UN mission in South Sudan (UNMISS) told AFP it was "extremely concerned" about the reported advance of the armed youths towards Bor, saying it must be halted "to avoid further bloodshed".
The Nuer fighters are members of a tribal militia known as the "White Army" that became synonymous with years of violence during the 1990s civil war.
The name is said to come in part from the ash the fighters use to protect their skin from insects.
In 2011 and 2012, the White Army turned on the Murle ethnic group, killing hundreds in a conflict over cattle theft.
"These youth have been reportedly moving across the state for some time now with the possible intention of attacking other communities," UNMISS spokesman Joseph Contreras said.
Claims that the White Army was being mobilised have cast a shadow over peace talks led by regional leaders to end two weeks of violence believed to have killed thousands. The unrest has also raised fears of a civil war.
Regional leaders at the Inter-Governmental Authority on Development (IGAD) have set Tuesday as a deadline for face-to-face talks between President Salva Kiir and his former deputy to end the battles for control over oil-producing areas, notably in the north of South Sudan.
While the government has said it was willing to observe a ceasefire, Machar, who was sacked as vice-president in July, has made demands including the release of political allies before committing to a truce.
