* Aid workers rely on donkeys after vehicles looted
* U.N. now treats over 1,000 malnourished children a month
* Ceasefire deals in January and May swiftly crumbled
* Rebels and government trade blame over continued fighting
By Carl Odera
LEER, South Sudan, July 21 (Reuters) - A surge in the number of emaciated children arriving at a feeding centre in Leer, a muddy rebel-held town in South Sudan's oil-rich Unity State, is fuelling fears that the world's newest nation is on the brink of famine.
Food stocks are running low across conflict-ravaged northern regions of the country, aid workers say, and the onset of the rainy season has dashed hopes that South Sudan's displaced subsistence farmers will plant enough crops to feed themselves.
The country is the size of France but has hardly any paved roads and the United Nations and humanitarian agencies are struggling to provide aid to remote regions. Even plane deliveries are dwindling as rain soddens dirt roads that act as air strips.
Aid agencies say South Sudan, which gained independence from Sudan in 2011 after decades of conflict, could be headed for the worst famine since the 1984 Ethiopian famine.
The charity Medecins Sans Frontieres (Doctors Without Borders) said it had treated about 1,800 malnourished children at its feeding centre in Leer since mid-May. In 2013 it treated 2,300 children during the whole year.
In relief agency feeding centres, mothers seeking medical help for skeletal children tell harrowing tales of life in the bush, where they could only scavenge for fruit or roots.
"We ran and hid in the bush for months without food," said one woman who fled fighting in the Leer region. "We were eating wild berries and plant roots."
Her three-year-old son weighs just 2 kg because of acute malnutrition, whereas a healthy three-year-old in South Sudan would normally weigh 10-12 kg, according to a paediatrician in Juba.
The United Nations has warned for months that a famine looms if there is no end to fighting that erupted in mid-December between the government and rebels. More than a third of the nation's roughly 11 million people could be on the brink of starvation by the end of the year, U.N. officials have said.
Aid agencies, who say funds to deal with the crisis are running low, complain work has been hindered by rebel and government soldiers who have looted vehicles and plundered food stores.
"We lost all our nine cars," said Sabrina Sharmin, an acting coordinator for Medecins Sans Frontieres (MSF), speaking in Leer last week. "Delivering (supplies) from the airstrip to our compound is challenging; we are now using donkey carts."
The U.N. Office for the Coordination of Humanitarian Affairs (OCHA) said aid agencies in Leer were treating more than 1,000 cases of child malnutrition every month. Before violence broke out in December it was about 40 per month.
Western diplomats say the looming famine is "man made" and avoidable. But they expect thousands to die and much of the country to suffer as neither President Salva Kiir or rebel leader Riek Machar appear ready to strike a lasting peace deal.
FOOD PRICES SOAR
South Sudan sits on Africa's third-biggest oil reserves and the United States and other Western backers hailed independence from Sudan three years ago as a foreign policy success.
But political turmoil and conflict threaten to plunge the country back into the same cycle of war and human misery that plagued its people when it was still part of Sudan and rebels were fighting the northern government in Khartoum.
At least 10,000 people have been killed and more than a million displaced since clashes between rival army factions broke out in December. Foreign investment has collapsed and the cost of the war has crippled the government's budget.
In Unity state, rebels loyal to former vice president Machar have seized oil wells and halted production.
At Leer market, a handful of makeshift shops sell millet, soap and even music CDs, a rare sign of normality in a town devastated by clashes between government and rebel troops.
But Kuel Nin, 34, a sorghum seller, said days would pass without customers purchasing anything. With roads blocked and river transport too dangerous as barges have often been attacked, prices of basic commodities in rural South Sudan have risen out of reach for many.
"It is expensive," said Nin, standing near a towering tree shading two abandoned and rusting military tanks, baking in the midday heat. "People suffer hunger, they have no money."
While there has been some small-scale crop planting in the last couple of months, Leer County rebel commissioner Peter Keak Jal said the effort was not enough to feed the population.
"We are approaching the end of the year so there will be no good food," Jal said. "It's going to be hard."
The U.N. World Food Programme declared a famine in the region in 2008 due to drought. Famine is defined as when acute malnutrition exceeds 30 percent of the population and the deaths exceed two people per 10,000 each day, among other criteria.
Rebels blame the government for violating two ceasefires agreed earlier this year and for driving the nation to disaster. The government accuses the rebels of sparking the conflict.
"It is of course the rebels to blame, they are now in flagrant violation of the cessation of hostilities," Information Minister Michael Makuei told Reuters in Juba, adding that only three of the 10 South Sudanese states would be affected.
Both sides have accused the other of violating two ceasefire deals - one in January and another in May.
The blame game holds little significance for Jared Tut. He fled Leer town after government troops attacked early in the year. He is now lying in an MSF-run hospital after diving into a river and being bitten by a crocodile.
"My house near the market was burned down (in the fighting) and I still cannot find my wife and child," he said in hospital.
(Writing by Drazen Jorgic; Editing by Edmund Blair and Susan Fenton)