Thousands of former rebels swore an oath of loyalty in a ceremony held under tight security in South Sudan.
Nearly 22,000 men and women -- drawn from the main parties as well as the South Sudan Opposition Alliance -- participated in Tuesday's proceedings, which were originally scheduled to take place in 2019 according to the peace deal.
South Sudan's President Salva Kiir Mayardit praised the progress made.
"Out of all the chapters of the agreement, the graduation of the unified forces under chapter two has been the most challenging and yet the most important", said President of South Sudan, Salva Kiir Mayardit.
"Over two hundred of your comrades have died in the eighteen training centres, some died because of disease and no medication, some died because of hunger as food was not available", added Riek Machar, Vice President and opposition leader.
The UN representative in the country also praised the new developments.
“A unified defence force is one of the most visible and meaningful expressions of national unity, especially in societies emerging from conflict. Its establishment will, on account of its inclusivity and diverse representation, symbolize the national identity of South Sudan", affirmed Nicholas Haysom, Special Representative of the Secretary General & Head of UNMISS.
The ceremony came weeks after the country's leaders -- appointed to run a transitional government -- announced that they would remain in power two years beyond an agreed deadline, sparking international concern.
Since achieving independence in 2011 from Sudan, the world's youngest nation has lurched from crisis to crisis, battling flooding, hunger, ethnic violence and political turmoil.
Go to video
DRC accuses Rwanda and M23 of planning attack on Goma
Go to video
At least 17 civilians killed by extremist rebels in eastern Congo
Senegal: Faction of Casamance rebellion inks peace deal
400 Commonwealth troops parade ahead of coronation of King Charles III
Sudan warring parties agree to 7-day truce from May 4
Helping young people find purpose through career guidance and sports (Inspire Africa)