JUBA (Reuters) - The president of war-ravaged South Sudan declared a unilateral ceasefire on Monday and promised to release political prisoners, but with no sign of a political deal with rebels it was not clear whether a truce would take hold.

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South Sudan has been mired in a civil war since 2013, when President Salva Kiir fired his deputy, Riek Machar. The conflict, fanned by ethnic rivalries, has sparked Africa’s worst refugee crisis since the 1994 Rwandan genocide and plunged part of the country into famine.

“I directed the prosecutor general to immediately review the cases of those who have committed crime against the state, commonly known as political prisoners, and ensure the necessary steps taken are taken to lead their release,” Kiir said in a speech in the capital, Juba.

“I am also declaring unilateral ceasefire effective from today.”

South Sudan analysts expressed scepticism that Kiir’s announcement would lead to long-lasting peace.

Kiir has declared ceasefires before and he has yet to release any political prisoners, said Alan Boswell, a South Sudan expert who authored a paper for the Small Arms Survey on the most recent failed peace deal in 2016.

Kiir’s speech offered no hint that he was willing to negotiate with the disparate rebel groups, the largest of which is led by Machar, Boswell said. Machar is an ethnic Nuer while Kiir is a Dinka.

In April 2016 the United States and other Western nations backed a peace accord that saw Machar return to the capital and again share power with Kiir. But the deal fell apart less than three months later and Machar and his supporters fled the capital, pursued by helicopter gunships.

Since then, the conflict has become increasingly fractured, with a patchwork of ethnic militias fighting in different parts of the oil-producing country.

“There’s no good incentive for the opposition movements to put down their guns because they’re not being offered any political settlement,” Boswell said.

The U.N. has warned the ethnic violence spill into genocide.