United Nations peacekeepers said they were concerned over claims thousands of armed youths from Machar's Nuer tribe were readying to attack Bor, the capital of Jonglei state, amid international efforts to stop two weeks of violence spiraling into all-out civil war.
"The forces of Riek Machar are now advancing on Bor, but we are confident we will hold them off and protect the town," army spokesman Philip Aguer said.
"There were heavy clashes at Gadiang north of Bor... the people in Bor are fearing an attack at any time."
Rebels were currently reported around 50 kilometres (30 miles) northeast of Bor.
It was not clear how many of the gunmen remained in the thick bush around Bor, but the army statement appeared to contradict claims by government spokesman Michael Makuei late on Sunday that "most of them have returned home".
Reconnaissance flights by the UN Mission in South Sudan (UNMISS) on Sunday identified armed groups but could not confirm the force's size.
The gunmen, a loose ethnic militia force loyal to Machar and dubbed the "White Army", are heavily armed – some carrying automatic rifles or spears, others armed with rocket propelled grenades. They are known for smearing white ash onto their bodies as war-paint and to ward off insects.
The world's youngest nation plunged into chaos on December 15 when President Salva Kiir accused his former deputy of mounting a coup, sparking deadly violence believed to have left thousands dead.
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 on 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."
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.
Grim reports of massacres, rapes and killings nationwide have emerged on both sides of the conflict.
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.


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