Machar, who under the terms of the September deal is to be reinstated as vice president, had not set foot in the city since he fled in July 2016 under a hail of gunfire when an earlier peace agreement collapsed.

The latest deal was signed in September to try to end a civil war that erupted in the world's youngest country in December 2013 and uprooted about four million people -- roughly a third of the population.

The rebel chief was welcomed by President Salva Kiir, Machar's former ally turned bitter enemy, on his arrival at Juba's airport from Khartoum.

The two rivals are to join regional leaders at the ceremony to publicly welcome the most recent agreement, signed in the Ethiopian capital Addis Ababa.

It was not immediately clear how long Machar would remain in Juba, as his aides have expressed concerns over his safety in the city.

- 'Here for peace' -

Lam Paul Gabriel, a spokesman for Machar's SPLM-IO rebel group, had said on Tuesday that he would be accompanied by around 30 political figures.

"We are worried for his security in Juba, but the truth is here: we are for peace, and what we are trying to do is build trust. So that is why he is able to leave his forces behind and just go with politicians," Gabriel said.

Several thousand people had already gathered for the ceremony at the John Garang Mausoleum, built in honour of the independence hero who was killed in a helicopter crash in 2005.

Among regional leaders in Juba for the ceremony were Sudanese President Omar al-Bashir, Ethiopia's newly appointed President Sahle-Work Zewde and Somalia's head of state Mohamed Abdullahi Mohamed.

Ugandan President Yoweri Museveni was also expected to attend.

Machar fled Juba in July 2016 after fierce fighting erupted between government forces and his rebels, leaving several hundred people dead.

He first headed on foot to the Democratic Republic of Congo before finally going into exile in South Africa.

- Deep humanitarian crisis -

South Sudan's civil war erupted when Kiir, a member of the Dinka tribe, accused his then deputy Machar, a Nuer, of plotting a coup.

The conflict split the country along ethnic lines and has seen mass rape, the forced recruitment of child soldiers and attacks on civilians.

It has caused one of the world's greatest humanitarian crises and wrecked the economy in a country which relies on oil production for the vast bulk of its revenues.

The United Nations and the African Union earlier this month appealed to the country's warring parties to make concrete steps to implement the latest accord.

South Sudan gained independence from its northern neighbour Sudan in 2011 after a 22-year civil war pitting rebel groups against Khartoum.

Several ceasefires and peace agreements have so far failed to end the fighting in South Sudan that has killed an estimated 380,000 people, uprooted a third of the population, forced nearly two-and-a-half million into exile as refugees and triggered bouts of deadly famine.

Sudan earlier this month appointed a peace envoy to South Sudan following the signing of the September accord in Addis.