John Kerry, US secretary of state, has urged South Sudan's government and rebel leaders to uphold a months-long promise to embrace a ceasefire or risk the spectre of genocide through continued ethnic killings.
Kerry, landing in the South Sudan capital of Juba on Friday, conveyed the threat of US sanctions against prominent South Sudanese leaders if the violence did not stop.
He also sought to compel authorities on both sides of the fight to put aside personal and tribal animosities for the good of a nation that declared independence three years ago to escape decades of war.
Now, South Sudan is engulfed in widespread killings that have largely broken down along ethnic lines and are drawing comparisons to genocide.
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It is estimated that thousands of people have been killed since the fighting began nearly six months ago, and about one million others have fled their homes.
If that continues, Kerry said on Thursday, it "could really present a very serious challenge to the international community with respect to the question of genocide".
"It is our hope that that can be avoided," he said on the eve of his daylong visit to South Sudan.
"It is our hope that in these next days, literally, we can move more rapidly to put people on the ground who could begin to make a difference."
The violence in South Sudan since last December is largely the result of ethnic tensions between the two tribes that boiled over when Kiir accused Machar of plotting a coup.
A month later, both sides agreed to a peace deal that eventually fell apart within days.
The US and UN are threatening to bring sanctions against both sides of the fighting - including, potentially, Kiir and Machar themselves.
Western officials are trying to persuade the African Union to deploy thousands of troops to South Sudan to keep the peace - or, as Kerry put it, make peace after massacres and bloody counterattacks show no sign of ceding.