The unification of forces loyal to President Salva Kiir and his rival, Vice President Riek Machar, was a key condition of the 2018 peace deal that ended the brutal five-year conflict in which nearly 400,000 people died.

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.

Earlier this month, South Sudan's leaders -- appointed to run a transitional government -- announced that they would remain in power two years beyond an agreed deadline, sparking international concern.

The transition period was meant to conclude with elections in December this year, but the government has so far failed to meet core provisions of the agreement, including drafting a constitution.

Nearly 22,000 men and women -- drawn from Kiir and Machar's 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.

The delays have fuelled frustration in the international community as explosions of violence threaten to undo even fragile gains.

As thousands of former rebels stood to attention at Juba's John Garang Mausoleum -- built in honour of South Sudan's independence hero who died in 2005 -- Chief Justice Chan Reech Madut administered the oath of loyalty.

Another 30,000 forces were also due to graduate in the coming days in training camps around the country.

In addition to joining the army and the police, the new graduates have also been integrated into the VIP protection force, the wildlife service, the civil defence and other organisations responsible for national security.

"These forces will be deployed all over South Sudan... After their graduation, the second batch will go for training," Information Minister Michael Makuei told AFP.

The ceremony was held under tight security, in the presence of representatives from the United Nations and neighbouring countries including Ugandan President Yoweri Museveni and Sudanese coup leader General Abdel Fattah al-Burhan.

It followed years of deadlock between Machar and Kiir over the division of senior posts in the unified armed forces command, with the two men only inking an agreement in April this year.

- Sticks not guns -

The addition of tens of thousands of former rebels to the government's payroll will add to already crushing economic challenges -- civil servants have been unpaid for months.

But the move was nevertheless met with optimism in some quarters, with one former rebel telling AFP he was excited to join the police force.

"I am looking forward to serving my people. I just want to tell our people that finally peace has come after a long struggle," said the former rebel who only identified himself as John, citing government restrictions.

Many of the new graduates carried sticks instead of guns at the ceremony, because of a years-long arms embargo imposed by the UN Security Council.

The government has repeatedly urged the UN to lift the embargo, even as deadly violence continues to roil the country.

"It is now high time for them to approve the purchase of the arms... because these security forces can not be security forces if they are not properly armed," said minister Makuei.

The UN has repeatedly criticised South Sudan's leadership for its role in stoking violence, cracking down on political freedoms and plundering public coffers.

It has also accused the government of rights violations amounting to war crimes over deadly attacks in the southwest last year.

The UN's World Food Programme warned in March that over 70 percent of South Sudan's 11 million people would face extreme hunger this year because of natural disasters and violence.

The United States last month pulled out of two peace process monitoring organisations in South Sudan due to the government's failure to meet reform milestones, citing a "lack of sustained progress".