After presenting the results of a detailed two-year inquiry to the media, Hollywood stars Clooney and Don Cheadle stopped by at the White House to discuss the crisis with President Barack Obama, a US official said.
South Sudan is the youngest country in Africa and support from the United States was instrumental in shepherding it into existence, but patience is running out with local leaders after it slipped back into civil war.
United Nations experts have blamed both Kiir and Machar's camps for reigniting the fighting and the world body is seeking to recruit forces for a stronger peace-keeping effort to protect civilians and aid workers.
But the report by The Sentry -- a watchdog group founded by Clooney and rights activist John Prendergast -- goes further in detailing the web of corruption sustaining and motivating the predatory behavior of local leaders.
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At his side, Prendergast warned that South Sudanese officials "no longer take seriously the threats made by the United Nations, by the United States and others to impose consequences for their behavior."
The report gives a detailed breakdown, backed by research on the ground and through international business networks, of the looting and describes a conflict that degenerated into a battle of resources between corrupt gangs.
Almost half of the population of South Sudan relies on food assistance for survival, and more than 800,000 South Sudanese have fled to neighboring countries as refugees.
The report said that while political rivalry is often blamed, the war's "key catalyst" is actually "competition for the grand prize -- control over state assets and the country's abundant natural resources -- between rival kleptocratic networks.
According to the report -- entitled "War Crimes Shouldn't Pay" -- this form of "competitive corruption" has dominated South Sudan's politics since long before the nation voted for independence in a 2011 referendum.
Both Kiir and Machar the report says, "benefit financially from the continuing war and have effectively ensured that there is no accountability for their human rights violations and financial crimes".