Rebels in violence-wracked South Sudan sought to take back control Monday of a key town they were forced from last week, the army said, vowing to defeat the militia.
United Nations peacekeepers said they were concerned 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 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 told AFP.
"Yesterday 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".
The gunmen, a loose ethnic militia force allied to Machar 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 faces as war-paint and to ward off insects.
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.
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.
There was also heavy fighting in the town of Malakal, state capital of oil-producing Upper Nile, but the army say they have now secured the town.
Grim reports of massacres, rapes and killings nationwide have emerged.
The UN says some 75,000 have sought refuge in badly overstretched peacekeeper bases across the country.