South Sudanese President Salva Kiir has arrived in Addis Ababa for peace talks aimed at brokering an end to the country's civil war, reversing an earlier decision as international threats of possible sanctions mount.

South Sudan's government and rebels are under intense diplomatic pressure to sign a deal by Monday to end a 20-month civil war in which tens of thousands of people have been killed.

Kiir previously said he would send his deputy to the talks in Ethiopia's capital after complaining it was not possible to strike an effective deal because rebel forces has split into factions.

Earlier this week, two South Sudan rebel generals said they had split with former vice president Riek Machar, rejecting his plans to join Kiir in a transitional government.

Elia Lomuro, South Sudan's cabinet affairs minister, said that Kiir would join the talks but it would not be possible to sign a full peace deal until all opposition factions could join the agreement.

"Riek Machar has already been ousted and disowned by his own army and politicians," Lomuro said.

"If we are to sign peace then we have to sign a peace with all the factions and all the groups that are fighting."

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Discussing the reported split, Al Jazeera's Charles Stratford, reporting from Adis Ababa, said: "We spoke to a rebel spokesperson here and he says the South Sudanese government is just using this as a pretext to delay the signing of this deal and that Machar is the only man that Kiir can speak to in terms of pushing these negotiations forward.

"They described the split of these two generals as an administrative problem.

"We believe that these two generals were sacked by Machar in July - it was only last week that they announced that they are leaving the rebels.

"The South Sudanese government spokesperson who we spoke to on Saturday said this was an indication of Machar needing to get what he described as 'his house in order' before these negotiations went any further."

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The latest round of peace talks opened on August 6, mediated by the regional eight-nation bloc IGAD (the Intergovernmental Authority on Development), as well as the UN, African Union, China and the "troika" of Britain, Norway and the US.

On Sunday, Ugandan President Yoweri Museveni, who has sent troops into South Sudan to back Kiir, held talks with Ethiopian Prime Minister Hailemariam Desalegn and Sudanese President Omar al-Bashir.

Kenyan President Uhuru Kenyatta is also in Addis Ababa for the talks.

"There is hope, as long as all of us want peace," Lomuro said, after reporters asked if a deal would be signed on Monday.

South Sudan's civil war began in December 2013 when Kiir accused his former deputy Machar of planning a coup, setting off a cycle of retaliatory killings that has split the poverty-stricken, landlocked country along ethnic lines.

More than 70 percent of the country's 12 million people need aid, while 2.2 million people have fled their homes, the UN says, with areas on the brink of famine.