Juba (AFP) - The United Nations says armed groups have been spotted close to a key town in South Sudan, after the government said thousands advancing on the state capital had been convinced to return home.
The UN has expressed concern over claims some 25,000 armed youths from the Nuer tribe allied to ex-vice president Riek Machar were readying to attack Bor, the capital of Jonglei state, amid international efforts to broker a ceasefire.
The world's youngest nation plunged into chaos two weeks ago when President Salva Kiir accused his former deputy of mounting a coup, sparking deadly violence believed to have left thousands dead.
Fuelled by ethnic rivalries between Kiir's Dinka group and Machar's Nuer, bloodshed has swept across the nation, with fierce battles reported in strategic oil-producing areas.
As fears soared over an impending attack on Bor by Machar's so-called "White Army", Juba said thousands of armed youths marching on the town had turned back.
Government spokesman Michael Makuei told AFP "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, unless" there is another mobilisation.
This is a positive development "because we do not want to lose more lives", he added.
The UN Mission in South Sudan (UNMISS) has been tracking reports of the armed groups' movements but cannot confirm the size or location of Machar's followers, said a UN statement.
"UNMISS today conducted aerial reconnaissance and reports that they have identified some armed groups approximately 50 kilometres (30 miles) northeast of Bor," said the statement.
However, it did not say who the groups were.
UNMISS chief Hilde Johnson, who is to brief the UN Security Council on Monday about the South Sudan crisis, has been seeking to persuade political and community leaders to head off the advance on Bor, the statement added.
According to Jonglei's acting governor Ogato Chan, the Nuer fighters were around 110 kilometres from Bor.
"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.
Fears over 'White Army' militia
The Nuer fighters are members of a tribal militia known as the "White Army" that became synonymous with years of violence and terror during the 1990s civil war.
Experts say the name comes 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 spearheaded by regional leaders to end the violence amid fears the country could slide into civil war.
Regional leaders at the Inter-Governmental Authority on Development (IGAD) have set Tuesday as a deadline for face-to-face talks between Kiir and Machar.
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 his arrested political allies before committing to a truce.
South Sudanese government spokesman Makuei told AFP earlier Sunday: "I really doubt if we, the South Sudanese government, will be in a position to sit with Riek Machar... He has not even respected the call by IGAD and the African Union to agree to the cessation of hostilities."
Fighting broke out on December 15 after Kiir accused Machar of mounting a coup, which his rival has denied. Rebels swiftly took over several key regional cities including Bentiu, in the northern oil-producing state of Unity, and Bor, which was later recaptured by the army.
In recent days, grim reports of massacres, rapes and killings nationwide have emerged.
The UN says the number displaced by the conflict has grown to 180,000 people and up to 75,000 have sought refuge in UNMISS bases in Juba, Bor, Bentiu, Malakal and Pariang.
South Sudan became independent in 2011 after a civil war that killed more than two million people between 1983 and 2005.