* Oil-rich nation is world's youngest country
* Five-year-old war has killed tens of thousands
* South Sudan president and rebel leader to meet again
South Sudan offered to allow a rebel representative to join its government on Friday, but ruled out Riek Machar, saying they had "had enough" of the rebel leader after five years of civil war.
"Machar cannot be part of government. We have had enough of him," Information Minister Michael Makuei Lueth told a news conference in the Ethiopian capital.
Despite that dismissal, Lueth said South Sudan's President Salva Kiir would meet his arch-rival and former vice president Machar for face-to-face talks again next week.
The two men met in Addis Ababa this week for the first time since a peace deal collapsed spectacularly in 2016, leading to the deaths of hundreds of people in fighting.
The war that broke out in 2013, less than two years after oil-rich South Sudan gained independence from Sudan, has killed tens of thousands and forced millions from their homes.
Footage released by the Ethiopian government showed Kiir and Machar shaking hands and hugging in a three-way embrace with Ethiopia's Prime Minister Abiy Ahmed this week.
Machar's rebel group SPLM-IO issued a statement calling the remarks from minister Lueth highly immature. "This bad politics is from a known peace spoiler and is only intended to derail the peace process," it said.
On Thursday the group condemned current efforts by regional bloc IGAD to convene meetings to discuss peace. "Workshops" will not stop the war and IGAD's overall model is "unrealistic", the rebels said.
Despite the mutual recriminations, Lueth said Kiir would meet Machar again next week in Khartoum, the capital of Sudan.
"Preparations are afoot to give a new impetus to this new round of talks and ensure a successful outcome," Sudan's Foreign Affairs Ministry said in a statement.
The ministry said next week's talks would be convened by Sudanese President Omar al-Bashir, a key player in South Sudan's history.
June 30 deadline
Before the people of South Sudan voted for secession and it declared independence in 2011, the region fought for more than two decades against Khartoum's forces, led by Bashir.
The international community, in particular the United States, played a major role in supporting the process that led to South Sudan gaining independence.
Last month the United Nations Security Council voted to renew some sanctions on South Sudan through mid-July and to consider imposing travel bans and asset freezes on six South Sudanese leaders unless the country’s conflict stops by June 30.
Minister Lueth said Machar's group was emboldened by sanctions against the South Sudan government and said the international community should levy measures against the rebels too.
Before arriving in Addis Ababa this week, Machar had been under house arrest in South Africa since 2016.
A spokesman for the rebel leader told Reuters Machar would travel to Khartoum on Sunday to attend the talks convened by Sudan.
Sudan is a member of IGAD, which has led the faltering peace process for South Sudan for several years. IGAD persuaded the warring sides to sign a ceasefire in December, but violence resumed within hours.