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South Sudan's leaders are responsible for creating a "government-made" famine, actor George Clooney wrote last week in an opinion piece in the Washington Post.

Co-authoring the op-ed with John Prendergast, Clooney took aim at the government of Salva Kiir in Juba, saying he is using the same tactics used by the Sudanese regime of Omar Hassan al-Bashir.

"In South Sudan today, war crimes pay," Clooney wrote.

"There is no accountability for the atrocities and looting of state resources, or for the famine that results."

The driving force of South Sudan's plight is the the "kleptocrats who have hijacked the government in Juba for their personal enrichment," Clooney said.

"A steep price should be paid for creating famine and benefiting from war," he added.

"Even while the world responds to the famine, it’s time also to address root causes and make those responsible pay for their crimes."

The United Nations is warning that the world is facing its worst humanitarian crisis since the end of World War Two, with more than 20 million people facing starvation and famine in four countries.

During his visit last week to South Sudan, the world's youngest nation, UN humanitarian chief Stephen O'Brien said he found a situation that is "worse than it has ever been."

"Parties to the conflict are parties to the famine – as are those not intervening to make the violence stop."

He said more than 7.5 million people need assistance, an increase of 1.4 million from last year. And some 3.4 million people are displaced, including nearly 200,000 who have fled South Sudan since January alone.