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International aid agencies slammed South Sudan's decision to raise foreign worker visa fees to as much as $10,000 (9,300 euros), warning it would worsen a humanitarian crisis in the famine-hit country, according to a statement published on Saturday.

"The government and the army have largely contributed to the humanitarian situation," said Elizabeth Deng of Amnesty International. 

"And now, they want to create profit from the crisis they have created."

The government measure, announced on March 2, would increase work permit fees for foreign workers from the $100-$300 range to between $1,000 and $10,000 per year, depending on the qualifications of the worker.

The measure could generate a revenue stream for the crisis-wracked nation, where oil revenues account for the near-totality of government earnings, but aid agencies said it could backfire.

"If this measure is put into practice, it will be impossible for humanitarian workers to pay this kind of sum," said Julien Schopp, director of humanitarian practice at InterAction, which groups 180 NGOs working worldwide.

Deng said there were hundreds of aid workers operating in the country, and that the new visa costs "could further hinder their critical work on the ground."

In late February, South Sudan - which formed in 2011 following a split from the north - declared famine in some regions.

On Saturday, the United Nations said more than 7.5 million people were in need of assistance there.

The UN and humanitarian organisations have described the crisis as having "man-made" origins, as a civil war begun in 2013 has forced people to flee, disrupted agriculture, sent prices soaring and cut off aid agencies from the worst-hit areas.

Aid agencies also say their workers have been subject to harassment and attacks, while the UN described the looting of "humanitarian assets".

On Thursday, Information Minister Michael Makuei said the new fees for foreign workers were already in effect and confirmed they applied to aid workers.

Schopp said NGOs were still pressing the government to provide details, notably on whether workers with current work permits would have to re-apply for new ones under the new fee structure.

On Saturday the UN's humanitarian chief, Stephen O'Brien, warned the world was facing its worst humanitarian crisis since the end of World War II in four countries - South Sudan, Nigeria, Somalia and Yemen.

War broke out in South Sudan in 2013, just two years following its independence, after President Salva Kiir accused his former deputy Riek Machar of plotting a coup.