Mediators who brokered South Sudan's fragile ceasefire called Tuesday for warring forces to abide by the deal, with bloodshed showing no sign of stopping despite promises made on paper.

"The parties should and must negotiate, and bring the fighting to an end," said Seyoum Mesfin, head mediator from the East African regional bloc IGAD.

Fighting broke out soon after dawn on Sunday, a day after the ceasefire was signed by President Salva Kiir and rebel chief Riek Machar, the second time in the five-month conflict that a truce has failed to stick.

Both sides have blamed each other for launching ground attacks and artillery barrages.

"All stakeholders must play their role and achieve a political solution," Seyoum told reporters in the Ethiopian capital, where dragging peace talks have made little if any progress to end a conflict that has sparked warnings of famine and genocide.

The ceasefire agreement, signed last week in Addis Ababa, was the fruit of weeks of mounting international pressure and shuttle diplomacy.

The war in the world's youngest nation has claimed thousands -- possibly tens of thousands -- of lives, with more than 1.2 million people forced to flee their homes.

Pagan Amum, former secretary general of the ruling party, who was arrested and put on trial for treason before being released as part of a peace gesture by Kiir, called on both sides to abide by the deal.

"Unfortunately, the agreement to immediately cease hostilities has not been respected," he said, after meetings with mediators.

Amum, one of a group of 11 political heavyweights who were detained after fighting broke out on December 15, is now trying to negotiate between the two sides.

"We call on the two leaders to cease all hostilities and respect the lives of our people, and allow humanitarian access in affected areas," Amum added.

The call to stick by the ceasefire follows warnings by UN Secretary General Ban Ki-moon of the possibility of a "special tribunal" to try those responsible for abuses in South Sudan.

In remarks to the Security Council, Ban cited a report by the UN mission in South Sudan which has said "there are reasonable grounds to believe crimes against humanity have been committed."

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