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South Sudanese President Salva Kiir and arch-foe Riek Machar are to meet in Khartoum on Monday for only their second round of talks in nearly two years, the Sudanese foreign ministry said.

The talks, which follow meetings in the Ethiopian capital on Wednesday and Thursday, come as both sides face a looming deadline to avert UN sanctions over their devastating four-and-a-half year civil war.

Tens of thousands of people have been killed and nearly four million driven from their homes since Kiir fell out with his then deputy in December 2013 dashing the optimism that accompanied independence two years earlier.

South Sudan's East African neighbours have warned that if their latest peace push is as abortive as previous ones, they will have little choice but to back sanctions.

But the omens are not good after Machar's rebel Sudan People's Liberation Movement-in-Opposition (SPLM-IO) described it as "unrealistic" and accused the government of planning a fresh offensive even before the talks opened.

"(Sudanese) President Omar al-Bashir will host direct talks between the two leaders of South Sudan in Khartoum on June 25," the foreign ministry said in a statement on Friday.

"Sudan will adopt all measures to ensure that the talks succeed," it added.

A preparatory team from the Intergovernmental Authority on Development (IGAD), the East African bloc under whose auspices the talks are being held, is expected to arrive in the Sudanese capital over the weekend.

It will be the first time Kiir and Machar have met in Khartoum since the outbreak of fighting in 2013.

What was discussed in this week's meetings in Addis Ababa has been kept under wraps, but Kenyan President Uhuru Kenyatta issued a downbeat statement after they opened.

"Over time our efforts have been eclipsed by the use of violence, giving credence to the call for punitive measures against perpetrators," Kenyatta said.

Since a 2015 peace deal collapsed in July 2016 with Machar fleeing to South Africa, Kiir's government has gained the upper hand militarily as the opposition has splintered into a myriad of factions.

Initially largely fought out between South Sudan's two largest ethnic groups - Kiir's Dinka and Machar's Nuer - smaller groups have since spawned their own militias raising question marks about the ability of either leader to halt the war.

Analysts warn that Kiir and Machar's notoriously volatile relationship and entrenched positions cast doubt on the likelihood of success of any reconciliation between them.

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