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Ben Curtis, File, Associated Press
In this Thursday, Jan. 2, 2014 file photo, 7-month-old boy Dhieu Ding Chol, left, and 5-month-old boy Thuch Jong Kuch, right, are held by their mothers as they receive treatment for dehydration, vomiting and diarrhea, likely caused by the lack of any sanitation where they are now living, at a clinic run by Medecins Sans Frontieres (Doctors Without Borders) set up in a school building in the town of Awerial, South Sudan.

NAIROBI, Kenya — More than 1 million people in South Sudan have fled their homes at a crucial time of the year: planting season. Famine, aid officials say, could be the result, and the U.N.'s top official for human rights said Wednesday she is appalled by the apparent lack of concern by the country's two warring leaders that mass hunger looms.

"If famine does take hold later in the year — and the humanitarian agencies are deeply fearful that it will — responsibility for it will lie squarely with the country's leaders, who agreed to a cessation of hostilities in January and then failed to observe it themselves," said U.N. High Commissioner for Human Rights Navy Pillay, referring to South Sudan's president and the former vice president.

UNICEF is warning that up to 50,000 children could die of malnutrition this year.

In a sign of how gravely the U.S. views the spiraling violence in the world's newest country, Secretary of State John Kerry will travel to Juba, the capital, this week as part of a multi-country trip through Africa. He said the U.S. government is closely considering levying sanctions against South Sudan's leaders, whom Kerry said are pursuing oil, money and power.

Fighting broke out in mid-December between supporters of President Salva Kiir and former Vice President Riek Machar. Thousands of people have died, including hundreds of people killed in ethnically targeted massacres.

Pillay told a news conference in Juba on Wednesday that South Sudan is on the verge of catastrophe. She said she was "appalled" at the apparent lack of concern about the risk of famine after meeting with Kiir and Machar.

The country faces a downward spiral, warned Peter Pham, the director of the Africa Center at the Washington-based Atlantic Council.

"It is a simple fact: This is planting season before the rains come, but people who have been forced from their homes or who, even if they remain at home for the moment, have a well-grounded fear of either not being there come harvest or having their crops stolen by ravenous bands of fighters, will not plant," Pham said.

That means a very lean period late this year, and an even more ominous period next year, he said.

Like the use of the word genocide, the use of the word famine is often seen as a measure of last resort. Pham said while there is no doubt that hundreds of thousands — "if not millions" — of South Sudanese will face grave food needs this year, he is also concerned that "sensationalist language aimed at grabbing headlines" could lead to desensitization.

Still, it's clear that world leaders have decided that South Sudan's situation is grave enough to warrant the use of the alarming word.

"Without immediate action, up to a million people could face famine in a matter of months," U.N. Secretary-General Ban-ki Moon said in mid-April. Rajiv Shah, the head of USAID, the U.S. government aid arm, said South Sudan "is on the brink of famine."

The World Food Program says it faces a $224 million shortfall of an emergency request made to feed 1.1 million people. A donor's conference is set for May 20. Even if funds are raised, WFP can't move food into areas of active conflict, noted Challiss McDonough, the WFP's spokeswoman in East Africa.

The conflict has put 7 million people at risk of hunger, said Toby Lanzer, the U.N.'s top aid official in South Sudan.

"April and May are the time to plant. April is behind us. Only May is left to enable people to prepare their fields and try to ensure that they have a harvest at the end of 2014," he said.

Nearly 250,000 children in South Sudan will suffer severe acute malnutrition this year if more is not done now, UNICEF, the U.N. children's agency, said last week. It said 50,000 children under the age of 5 are likely to die.

"These are not mere statistics — they are the children for whom South Sudan holds so much potential and promise," said Jonathan Veitch, the UNICEF representative in South Sudan. "We must not fail the children of this new and fragile nation."