The United Nations on Tuesday pushed South Sudan's rival sides to quickly form a new unity government after rebel leader Riek Machar returned to Juba and was sworn in as vice president.

Machar's return on a UN plane marked an important step in the international effort to force the rebel and government sides to implement a peace accord that was signed in August but has yet to take hold.

The peace deal is meant to end a two-year war that began with the falling out between President Salva Kiir and Machar, who served as vice president when South Sudan won independence in 2011 to his dismissal in 2013.

The Security Council urged both sides to "quickly form the transitional government and fully implement the peace deal," said Chinese Ambassador Liu Jieyi, this month's council president.

"There is much work to be done to bring peace and stability to South Sudan," said Liu following a council meeting to take stock of the way forward to restore peace in South Sudan.

UN Secretary-General Ban Ki-moon said the rebel leader's arrival in Juba opened up "a new phase in the implementation of the peace agreement" and called "for the immediate formation of the transitional government of national unity," said a statement from his spokesman.

Under the agreement, Machar will serve alongside Kiir in a new 30-month transitional government leading to elections.

UN peacekeeping chief Herve Ladsous told the council that Machar's return "should open a new chapter" and "allow the real transition to begin."

South Sudan Deputy UN Ambassador Joseph Moum Malok said the new transitional government should be formed "in a day or two after consultations with the different parties in the country."

"It's vital that the parties take this opportunity to show their genuine determination to move forward with the peace process," said Ladsous.

- Turning up the heat -

US Ambassador Samantha Power said that while Machar's return marked an important step, international powers remained "clear-eyed" about the challenges ahead.

"It's the best hope South Sudan has had in a very long time," Power told reporters, but she added that the council must keep pressure on Machar and Kiir to implement the peace deal.

The new government will have to tackle security sector reform to end fighting, corruption and, in particular, pick up the pace on plans to set up a special African Union court to try war crimes suspects, said Power.

"When things are going to happen in South Sudan, it tends not to happen because of gravity. It happens because the international community unites and turns up the heat," she said.

South Sudan's war began in December 2013, when Kiir accused Machar of plotting a coup.

The conflict has torn open ethnic divisions and been characterized by horrific rights abuses, including gang rapes, the wholesale burning of villages and cannibalism.

Tens of thousands of people have been killed and more than two million have been driven from their homes in the violence that erupted just two years after South Sudan won independence.

Machar had been expected in the capital on April 18 from his base in the east of the country, but last-minute disputes over the security arrangements, including the weapons that rebel troops would be allowed to carry, led to delays.