South Sudan's rebel chief Riek Machar finally returned to Juba on Tuesday and was sworn in as vice-president of the world's newest country, calling for "unity" after more than two years of ferocious civil war.

"We need to bring our people together so they can unite and heal the wounds," said Machar, greeted by ministers, diplomats and the release of white doves as he stepped out of a UN plane, after a week-long delay that had threatened a long-negotiated peace deal.

Machar, who was originally due back on April 18, headed immediately to the presidential palace to be sworn in alongside his longtime arch rival, President Salva Kiir.

Kiir, who shook the hand of Machar and called him "my brother", said they would "work immediately" to set up a unity government.

"I am very happy to welcome and warmly receive my brother Dr. Riek Machar," Kiir said. "I have no doubt that his return to Juba today marks the end of the war and the return of peace and stability to South Sudan."

UN Secretary-General Ban Ki-moon said Machar's return marked "a new phase" in efforts to seal peace and called for "the immediate formation" of a unity government."

The peace deal struck in August 2015 to end the brutal war provides for a 30-month transitional government leading to elections.

Machar's delay in returning to Juba under the terms of the deal had infuriated the international community after months of negotiations spent on getting the rivals to return to the city and share power.

"I am very committed to implement this agreement so that the process of national reconciliation and healing is started as soon as possible, so that the people can have faith in the country that they fought for, for so long," Machar said on being sworn in.

- Deep suspicion -

Ensuring they work together in a unity government, and that the thousands of rival armed forces now in separate camps inside the capital keep their guns quiet, will be an even bigger challenge.

Both sides remain deeply suspicious, and fighting continues with multiple militia forces unleashed who now pay no heed to either Kiir or Machar.

Machar's return had been stalled by arguments that at one point, in a country awash with weapons, came down to a dispute about just over two dozen rocket-propelled grenades and machine guns that the force guarding him were allowed to have.

Tens of thousands of people have been killed and more than two million driven from their homes in the conflict, which has reignited ethnic divisions and been characterised by gross human rights abuses.

The economy is in ruins, over five million people need aid and over 180,000 people are crammed into UN peacekeeping camps, too terrified to venture outside the razor wire fences for fear of being killed.

Tensions are high, and the days ahead will be critical.

"We need the guns to stay silent and give people time -- both as official warring parties and as individuals -- with one another in coming days," said Casie Copeland from the International Crisis Group (ICG) think tank.

Suffering is on an epic scale. Parts of the country, especially the devastated oil producing northern Unity region, have been pushed to the brink of famine.

There are huge expectations Machar's arrival means the myriad of problems will be solved swiftly -- but there will be no quick fix.

- 'Best chance yet' -

Diplomats note gloomily that while Machar's return is the "best chance yet", the deal imposed under intense international pressure only sees the country go back to the status quo that existed before his July 2013 sacking as vice president that precipitated the war.

The agreement has already been repeatedly broken with months of fighting since it was signed, and its key power sharing formula left in ruins after Kiir nearly tripled the number of regional states.

The fighting erupted in December 2013 when Kiir accused Machar of plotting a coup.

The conflict has witnessed the abduction and rape of thousands of women and girls, massacres of civilians, recruitment of child soldiers, murder, mutilation and even cannibalism.

South Sudan is one of poorest countries on the planet, and had some world's worst indicators for development, health and education even before the war.

Machar has over 1,500 armed troops in the capital, while government forces have officially just over double that number.

All other soldiers have to remain at least 25 kilometres (15 miles) outside the capital.

The threat of violence at a local level remains enormous, with multiple militia forces unleashed and out of control.

Machar and Kiir are decades-old rivals and even if they can work together both must also rein in powerful hardline field commanders.