As he approaches the imposing compound on the outskirts of Juba, a motorbike taxi driver shakes his head and refuses to stop within 200 metres (yards) of South Sudans military headquarters.

He says the fear of crossing the security forces at night is what empties the busy streets, where by day hundreds of his colleagues loiter on corners vying for business.

Inside the base of the Sudan Peoples Liberation Army (SPLA), a rebel force that fought a guerrilla war to win independence, the tense atmosphere and suspicious gaze of fighters match the level of public mistrust found outside the heavy-walled compound.

"The SPLA has been fighting a war for the last 21 years. We had an army with a revolutionary mandate, a liberation mandate from the people of South Sudan to fight that war," which has now expired, said military spokesman Philip Aguer.

South Sudan became the worlds newest nation on July 9 after southerners voted almost unanimously in a January referendum to secede from the north.

Now the government is keen to professionalise the huge army and reintegrate the unknown number of men, women and children who spent years fighting in the bush back into a society that has very few employment opportunities.

"SPLA will be transformed into the South Sudan Armed Forces," said Aguer, warning that a name change is just the first step in a long and delicate process of keeping the peace, while phasing out masses of fighters who sacrificed everything for their country.

"We are trying to transform into a modern army, but our problem is the issue of the integration of rebel militias," said Mac Paul, deputy director of South Sudans military intelligence.

While South Sudan has technically been at peace since a 2005 agreement with its former civil war enemy, tensions are escalating.

As north and south trade accusations of funding rebel militia groups inside each others resource-rich border states, their fragile economies are further destabilised and the outbreak of a new war appears more likely.

Paul said the army has to abide by a government strategy for peace that started in August when President Salva Kiir offered an amnesty to rebel groups willing to lay down their arms.

This has drawn many renegade SPLA leaders and their forces back into the fold but has also further bloated military spending that Lam Akol, the leader of the SPLM for Democratic Change opposition party, says exceeds 40 percent of the peacetime budget.

Akol says the money is desperately needed to build a new nation from scratch and has warned that if people do not "see the fruits of peace" through basic service provision they will rise up.

But Finance Minister Kosti Ngai says that while "expenditures for security might seem to be unreasonably high," they are "a matter of life and death" for the fledgling nation.

As part of the military transformation, and in a bid to reduce spending, the government has pledged to demobilise 80,000 soldiers and 70,000 members of other uniformed forces such as the police and wildlife service under a UN demobilisation, disarmament and reintegration (DDR) programme.

But critics say the exercise is futile until South Sudan sees real peace, as the number of rebels absorbed will equal those demobilised, and the army will continue putting only low-ranking officers or ancillary staff through the programme.

"Unless you have an analysis of the SPLAs composition and capabilities and an overarching plan, then transformation efforts will always be frustrated, including DDR," said Richard Rands, a former special forces officer who has trained around 7,000 SPLA soldiers since 2006.

He says the force has come a long way, but international expectations that its tranformation can be completed in three to five years are wildly otimistic.

On a dusty field outside an SPLA base in Bentiu, capital of Unity state, a rag-tag bunch of men turn on their heels, march and stomp at the command of a handful of officers in military fatigues.

The oil-rich state on the border with the north is one of the most active for rebel militias, and the 4th Division stationed there is growing as it tries to reduce a steady flow of attacks.

Major General Mangar Buong says the men milling about the courtyard are the latest recruits, following an October attack in the state's Mayom county that killed around 80 people, including 11 civilians.

"The ones cleaning here now, these are the ones surrendering," who were just farmers lured by the promise of goods and guns, Buong said.

He reiterates the south's constant refrain that it is the Khartoum government that is behind the rebel militias and says the army will keep taking in defectors as long as the north keeps using them.