Ethiopia, which has helped broker the meeting, says rebel leader Riek Machar, who fled South Sudan in July 2016, is expected to meet face-to-face with the country's president, Salva Kiir.

Machar arrived in the Ethiopian capital Wednesday morning for the talks, Menasseh Zindo, a senior official in his Sudan People's Liberation Movement-in Opposition (SPLM-IO) rebel group, told AFP. A foreign ministry spokesman confirmed his arrival.

The official scope of the talks is broad -- to build bridges between the two.

But analysts say the outcome remains unclear given their notoriously volatile relationship, and there is doubt whether the meeting will even take place.

Once comrades-in arms in the fight for independence, Kiir and Machar experienced a bitter falling out, a development that played a key part in the civil war that blights the future of the world's youngest state.

Tens of thousands of people have been killed and nearly a third of the 12 million population have been driven out of their homes, and many to the brink of starvation.

The two will meet at the invitation of Ethiopia's new prime minister, Abiy Ahmed, who also chairs the Intergovernmental Authority on Development (IGAD) regional bloc that has taken the lead in thus-far fruitless peace negotiations.

Abiy "will call upon the two leaders to narrow their gap and work for the pacification of South Sudan and relieve the burden of death and uprooting of South Sudanese people," Meles Alem, Ethiopian foreign ministry spokesman, said.

Kiir's attendance has been confirmed by South Sudan's ambassador to Ethiopia, James Pitia Morgan.

A landlocked state with a large ethnic mix, South Sudan gained independence from Sudan in 2011 after a long and brutal war.

The event was hailed around the world and by celebrity supporters such as George Clooney.

But in 2013, Kiir accused Machar, his vice president, of plotting a coup against him, and violence erupted between the two factions, feeding on brooding ethnic tensions.

- New fighting -

They have not met since July 2016, when heavy fighting in the capital Juba signalled the collapse of a 2015 peace deal and Machar fled to South Africa.

The renewed violence spread across the country, spawning numerous new armed opposition groups and further complicating peace efforts.

Despite the pressure, observers say Kiir has little incentive to make concessions to his rivals.

His soldiers are winning militarily, while the opposition is more fractured than ever before.

Efforts to revitalise the 2015 agreement resulted in a ceasefire in December which lasted just hours before warring parties accused each other of breaking it.

The meeting in Addis Ababa comes against a background of growing international frustration.

In May, the UN Security Council gave the two warring sides a month to reach a peace deal or face sanctions.

Washington was a critical backer of South Sudan during its separation from Sudan, and remains Juba's biggest aid donor.

But a top US official earlier this month threatened parties on both sides of the conflict with sanctions after a report from a US foundation, The Sentry, said South Sudanese elites were profiting from human rights abuses.