South Sudanese refugees line up to receive a lunch of maize mash and beans at the Imvepi reception center on June 6 in Northern Uganda. (Ben Curtis/AP)

The top U.S. official for humanitarian aid has delivered a stern warning to South Sudan's president that the Trump administration is reexamining its policy toward one of the world's poorest and most dangerous countries as the African nation slides into lawlessness.

Mark Green, the administrator for the U.S. Agency for International Development, met Friday with President Salva Kiir. Green said he raised U.S. concerns over the dangers humanitarian aid workers face in delivering food and medicine in the country as well as a pervasive climate of criminal activity by government forces, criminal gangs and opposition forces.

Since civil war erupted almost four years ago, a third of South Sudan's population has become internally displaced or fled the country in Africa's worst refugee crisis since the 1994 Rwandan genocide.

"I told him we are, in the next few weeks, undertaking a complete review of our policy toward South Sudan," Green said in an interview immediately after the meeting on Friday, which was embargoed for security reasons until he left the country the next day.

USAID officials said the policy review will be comprehensive, though not as formal as reviews conducted over such troubled nations as Afghanistan.

Although Green said he did not mention specific consequences if U.S. concerns are not addressed, like sanctions or an arms embargo, he considered the message clear.

Green called South Sudan "a very dangerous place in which we're seeing atrocities occur all the time. And while it is true that we support the people of South Sudan, it is just as true that the situation has deteriorated to the point where a serious reexamination of U.S. policy is appropriate."

[South Sudan’s people are starving, and fighters are blocking aid]

Alarm has been growing over deteriorating conditions in South Sudan, the world's newest country after it declared independence in 2011. A civil war has devastated the country since it started in December 2013, following a dispute between Kiir and Vice President Riek Machar, political rivals who head the Dinka and Nuer ethnic groups. Government troops have won back much territory over opposition forces, but fighting involving many splinter groups continues. That has made it more difficult for aid workers — who often negotiate with a dozen or more groups, some of dubious authority — to deliver truckloads of food and supplies overland.

In the chaos, a senior U.N. official warned last month that South Sudan is on the precipice of an "impending abyss."

The war has bankrupted the government, despite revenue from its oil fields and fertile land that once made this a breadbasket for the Horn of Africa. It lacks salaries to pay its soldiers and civil employees. Unpaid soldiers and criminal gangs own the night in the capital of Juba, looting homes. In rural areas, soldiers have reportedly stolen crops from farmers. Checkpoints are manned by armed men who demand a "tax" to pass. Humanitarian groups are frequently held up by bandits.

Everyone seems to be suffering but the political and military elite. A report last year by a George Clooney-funded project, the Sentry, said South Sudan is run by a "kleptocracy" that has enriched itself by profiteering.

Human rights groups, U.N. humanitarian aid officials and many governments blame both the government and opposition forces for preying on civilians.

"All sides are guilty of human rights abuses," said an aid worker with long experience in the country. "The fabric of society is broken. You can't imagine it will keep going down, and then it goes further down."

With no bottom in sight, international organizations and donor nations recently have dispatched officials to South Sudan to urge that it take part in the High Level Revitalization Forum, a regional peace initiative that aims to revive a stalled 2015 peace agreement.

[Europe is trying to cut the flow of migrants from Africa. It won’t be easy.]

"The international community is in a difficult situation," said David Shearer, head of the U.N. Mission in South Sudan. "Because on the one hand, there doesn't seem to be an immediate end to the fighting and the conflict. On the other hand, there are millions of people who need aid. And to just pull that aid will mean thousands of people will die."

As head of USAID, Green embodies U.S. humanitarian aid to South Sudan. This fiscal year, the United States spent more than $500 million getting emergency aid to the 2 million internally displaced South Sudanese. Counting aid to another 2 million refugees in neighboring countries, this year the United States spent almost $730 million, making it by far the biggest donor.

U.S. efforts to push an arms embargo on South Sudan have failed at the Security Council. But Washington's leverage on the government in Juba is limited by its unwillingness to walk away from the 6 million Sudanese — more than half the 10 million left in the country — who are considered "food insecure."

During their Friday meeting, Green said he sought to impress on Kiir the impatience and broad frustration Washington is feeling over his handling of the conflict and the now-normal level of violence.

"Part of what I wanted to do is to make sure the government of South Sudan appreciates, yes, there's a new administration," he said. "But this is not a partisan issue. This is an issue in which everybody I know from the U.S. government and the U.S. political scene is on the same page."

But Green said Kiir disputed every concern he raised. According to Green, Kiir insisted that there is no systemic insecurity in the country, that lies are spread by the opposition, which he blames for cease-fire violations, and that humanitarian access is unfettered.

"He would say it's either the opposition's fault, or he would say that the situation isn't as bad as I've heard," Green recalled. "And I said, 'I respectfully disagree, because I have heard from so many people. And I said, as the leader of America's largest aid agency, we have people in the field and partners in the field. Their security is of paramount concern.' "

Green spent Saturday in Wau, South Sudan's second-largest city, where he was struck by the disconnect between Kiir's dismissal of U.S. concerns and what he observed. A United Nations compound in this once-quiet city is encircled by a makeshift camp filled with 32,000 frightened civilians who prefer sleeping on rainy-season mud floors under peacekeeper protection than going home to neighborhoods where the sound of shooting ricochets through the night.

He also visited the city's Catholic cathedral, one of four churches in Wau where thousands more civilians have sought refuge. Humanitarian workers at the two sites said the residents are overwhelmingly ethnic minorities with homes nearby and are considered supporters of opposition groups. Their fears are of government forces, not the rebels who have a front about five miles outside town, the aid workers said.

A schoolteacher named James, who has been elected as a leader in the U.N. compound, expressed despair when Green asked him why people don't go home.

"Where would we go?" he told Green. "We will be killed. We would really like to go back where we were. But we are scared."

After departing Wau, Green said that what he observed and heard in Wau contradicted every denial Kiir made.

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