South Sudan's warring rivals President Salva Kiir and rebel leader and former vice president Riek Machar will attend a retreat at the Vatican this week "to work for a future of peace", the Holy See said.
The arch-rivals will attend "an occasion for encounter and reconciliation, in a spirit of respect and trust," the Vatican said in a statement, describing it as "both ecumenical and diplomatic at the same time".
The encounter on Wednesday and Thursday between "those who in this moment have the mission and the responsibility to work for a future of peace and prosperity for the South Sudanese people" had been proposed by the Archbishop of Canterbury Justin Welby, who will also attend.
Kiir left for the retreat on Tuesday, government spokesman Ateny Wek Ateny told journalists in Juba, adding that Machar too had already travelled from Khartoum, where he has been living in exile.
The two men last saw each other in October last year, shortly after the signing of a power-sharing deal, when Machar made a brief return to Juba for the first time since fleeing on foot in a hail of gunfire in July 2016.
Machar was Kiir's vice president until a falling out in 2013 which kickstarted a civil war just two years after independence from Sudan.
Battles between those from Machar's Nuer community and Kiir's Dinka people were characterised by brutal violence, rape and UN warnings about "ethnic cleansing".
In 2015 a peace deal was signed that saw Machar return as vice president in 2016, but the deal collapsed within months, with fierce battles in the capital. The fresh conflict engulfed even more of the country.
Machar fled to the Democratic Republic of Congo, and went into exile in South Africa and later Sudan which played a key role in the latest peace deal.
The power-sharing agreement will again reinstate Machar as vice-president, and the unity government is meant to be in place in May.
However, observers warn implementation of the deal has stalled.
Crucially, the two men have yet to negotiate security control of the capital, moves to establish a national army, and other sticking points.
Several ceasefire agreements and peace pacts have failed to end the fighting, which has led to the death of an estimated 380,000 people, uprooted a third of the population, forced nearly two-and-a-half million into exile, and triggered bouts of deadly famine.
Pope Francis has repeatedly voiced concern for South Sudan.
He granted an audience to Kiir last month and afterwards said he hoped to visit the country to "encourage the peace process."
Participants will receive a Bible at the end of the retreat with the message "Seek that which unifies. Overcome that which divides," the Vatican said.