Machar - whose return was delayed after a row last week over the weapons he could bring with him - was sworn in as first vice president alongside his longtime rival, President Salva Kiir, in the capital Juba on Tuesday.

Machar was driven immediately from the airport to the presidential palace to be sworn in, spokesman William Ezekiel said.

The United Nations peacekeeping chief Herve Ladsous told the Security Council on Tuesday: "The return of the designated first vice president should open a new chapter for the country. It should allow the real transition to begin." The 15-member council met to discuss the two-year war in South Sudan just as Machar was sworn in.

Under the peace deal, Machar will serve alongside Kiir in the new 30-month transitional government leading to elections. "It's vital that the parties take this opportunity to show their genuine determination to move forward with the peace process," Ladsous said.

Chief of the rebel military Simon Gatwech Dual had returned to the country on Monday. "We are one South Sudan," Dual shouted, reportedly waving a walking stick in the air as he marched off the plane.

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."

Fragile peace

The return is a step towards peace in the war-ravaged country. South Sudan's civil war began in December 2013 when Kiir accused Machar of plotting a coup.

The success of the unity government is not a given, based as it is on hopes that the thousands of rival armed forces in separate camps inside the capital keep their guns quiet. Both sides remain suspicious of each other and fighting continues, with some militia forces who listen to neither Kiir nor Machar.

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

The international picture

Machar's delayed arrival angered the international community after months of negotiations on getting the rivals to return to the same city and share power in the world's newest country.

A bitter conflict

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 characterized by gross human rights abuses.

The economy is in ruins, over five million people need aid and two million have fled their homes.

jbh/jm (Reuters, AFP)