The South Sudanese army, loyal to President Salva Kiir, said Monday it had begun withdrawing from the capital Juba ahead of a peace deal deadline agreed with the rebels of former deputy president Riek Machar.

The peace accord, signed on August 26, requires the withdrawal of all military forces within a 25-kilometre (15-mile) radius of the city within 90 days, a period which ends later this week.

But the late start of the army pull-out means a full withdrawal won't be completed by the agreed date.

The South Sudanese army "is implementing the security arrangements and the ceasefire agreement that stipulated the redeployment of forces from Juba to a distance of 25 km outside Juba town," army spokesman Colonel Philip Aguer told reporters.

"This has started today by SPLA (South Sudan's national army) headquarters redeployment of ground forces from Juba, starting with 250 that are heading to Mogiri" 25 kilometres northeast of Juba, he added,

Aguer assured that the army "is 100 percent committed to the peace agreement and particularly to the ceasefire".

However he added that the troop withdrawal would not be completed in the allotted time.

"Most of the process of redeployment of forces, creation of camps, should have been completed by the end of December," he added.

South Sudan's latest civil war began in December 2013 when Kiir accused his sacked deputy Machar of planning a coup. The fighting quickly spread beyond the capital and has been characterised by ethnic attacks, massacres of civilians, recruitment of child soldiers, rape and other war crimes.

The conflict has triggered a humanitarian crisis with 2.3 million people forced from their homes and 4.6 million in need of emergency food. Tens of thousands have died and the economy has all but collapsed.

The peace deal also foresees a 30-month "transition period" with the two sides sharing power.

Elections would then follow, theoretically in early 2018.

But so far the rebels have failed to send any delegates to Juba, delaying the political side of the deal.

South Sudan is the world's youngest nation, having ceded from Sudan and proclaimed its independence in July 2011.