South Sudan's warring leaders failed to reach a deal to end more than a year of civil war, mediators said Friday, with the latest collapse in peace talks paving the way for possible sanctions.

Ethiopia's prime minister said South Sudan's President Salva Kiir and rebel leader Riek Machar missed a deadline to reach a peace agreement by midnight Thursday, and that further talks on Friday "did not produce the necessary breakthrough."

"This is unacceptable, both morally and politically," Hailemariam Desalegn said in the statement issued by the east African regional bloc IGAD, which has been trying to mediate a peace deal.

Hailemariam also gave IGAD's harshest criticism yet of Kiir and his former deputy Machar, whose personal feud has exploded into ethnic massacres, gang rapes and the forced displacement of civilians, pushing the country to the brink of famine.

"Continuing a war flagrantly disregards the interests of you, the people," he said, addressing the people of South Sudan, whose country only gained independence from Khartoum in 2011 after a long, bitter war.

"It is an abdication of the most sacred duty leaders have to you, their people: to deliver peace, prosperity and stability," the Ethiopian premier said of Kiir and Machar, both of who have been implicated in atrocities.

"I asked them to be courageous in offering compromises and alternatives, rather than only reiterating old positions.... Unfortunately, as the missed deadline shows, our pleas have not been heeded."

South Sudan's civil war started in December 2013 when Kiir accused Machar, who had been sacked as vice president, of attempting a coup.

Over two dozen armed forces -- including government soldiers and allied militia backed by Ugandan soldiers on one side, and a range of rebel factions on the other -- have been battling it out since.

Tens of thousands of people have died in the conflict, two million have been uprooted and 2.5 million are in desperate need of food aid.

In a bid to force a deal, the United Nations this week passed a resolution threatening sanctions against individuals deemed to be undermining peace efforts.

Possible targets include leaders or officials who obstruct peace talks, impede humanitarian aid deliveries, recruit child soldiers or attack UN peacekeepers.

Acknowledging IGAD's failure to broker peace, Hailemariam said the "peace process must be reinvigorated and reformed".

"We will assist the parties to make the compromises that have so far eluded them. We will use all influence at our disposal to convince those that remain intransigent," he said, alluding to mounting calls for sanctions and an arms embargo.

According to diplomats close to the peace process -- which has so far cost at least 20 million euros and earned the peace delegates scorn for drawing out their stays in luxury hotels -- IGAD is likely to pull in the weight of the African Union, the 54-member pan-African bloc.

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