BIDI BIDI, Uganda (Reuters) - Besides bags, blankets and tales of horror, some of the thousands of refugees fleeing South Sudan's civil war each day carry something else - the ethnic hatred the United Nations says is "fertile ground" for genocide.

That hatred, fueled by continuing reports of ethnic-based killings inside the country, is turning refugee camps on its borders into tinder-boxes and threatening to destabilize the wider region.

More than a million people have fled the world's youngest nation since fighting erupted in late 2013, the biggest cross-border exodus from any central African conflict since the 1994 Rwandan genocide.

They are going in all directions, including Ethiopia, Democratic Republic of Congo and Sudan, but Uganda, directly south, has received the most - 600,000 refugees so far.

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Bidi Bidi, a parched 250 square km expanse of scrubland in northern Uganda, was only set up as a camp in August. It is already home to 260,000 South Sudanese, and authorities say that during November 2,700 more arrived every day.

Earlier this month, the desperation and anger of its residents boiled over into confrontation as Dinka women from South Sudan's largest and dominant ethnic group were attacked as crowds of refugees waited to receive food.

Vicky Nyoka, a 39-year-old refugee who witnessed the incident, said the trouble kicked off when young men started shouting that the Dinka were "the root cause" of their suffering. A mob then turned on the women.

"One Dinka woman, she was beaten badly and her arm was broken," Nyoka told Reuters. Three others were injured, she said, before security guards intervened, firing tear gas and warning shots to quell the confrontation.

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Robert Baryamwesiga, head of what is fast becoming one of the world's largest refugee camps, fears the flare-up will not be the last, especially if the ethnic cleansing recorded by the United Nations continues.

"What is happening over in South Sudan affects the relationships of refugees in the settlement a great deal," Baryamwesiga told Reuters.

MACHETES

South Sudan gained independence from Sudan in 2011, the final act of a long deal to end decades of civil war between the largely Arab, Muslim north and the predominantly African, Christian or animist south.

However, civil war broke out in December 2013 when President Salva Kiir, a Dinka, fired Riek Machar, a Nuer, as his deputy. A peace agreement brokered last year fell apart in July when heavy fighting erupted in the capital Juba between Kiir and Machar forces.

All of Bidi Bidi's residents arrived since July, and the U.N. estimates 300,000 more people will enter Uganda next year as the fighting spills south and west from Juba, laying waste to towns and villages and uprooting their inhabitants.

"What we're seeing is an already dire situation getting even worse," said Charlie Yaxley, spokesman for the U.N. refugee agency UNHCR in Uganda.

Machar himself was injured in the July fighting and eventually made it to South Africa for medical treatment, where he is now being held as a "guest" of Pretoria to keep him out of circulation, according to a diplomatic source.

The United Nations Human Rights Council has reminded Kiir's government of its responsibility for protecting the population against genocide and the government has said it will let a 4,000-strong regional protection force bolster the U.N.'s existing peacekeeping mission.

The head of the council said on Wednesday that force should be deployed straight away. "South Sudan stands on the brink of an all-out ethnic civil war, which could destabilize the entire region," commission chief Yasmin Sooka told an emergency session of the council in Geneva.

Most of those arriving in Bidi Bidi are from smaller tribes in the southern part of the country, and have personal accounts of ethnic pillage, rape and murder - in many cases at the hands of Dinka militiamen.

Awate Dawa, a 56-year-old widow and mother of four from a village in Yei River state, abutting Uganda, said Dinka soldiers armed with machetes came at night in late July and ordered her neighbor to come out of his house.

"They started cutting him until he died," said Dawa, a member - like her slain neighbor - of the Kakwa tribe.

Throughout the attack, the soldiers kept threatening to kill all Kakwa for their alleged support of Machar's rebels, she said.

Kiir told Reuters in December there was "no ethnic cleansing" in South Sudan.

SPEAK DINKA, AND LIVE

Every night, buses pull into Bidi Bidi's reception areas, flinging open their doors to reveal dozens of haggard and hungry boys and girls, or women, young and old, cradling babies. Among them sit a handful of middle-aged men.

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Some say they are simply victims of the lawlessness and banditry that has worsened since war broke out three years ago, but many say they were attacked by government forces after being accused of supporting Machar's Nuer.

Some are Dinka, also fleeing attacks, but Baryamwesiga said there were not many Dinka in the Bidi Bidi camp.

Joyce Poni, a 31-year-old from the Pojulu tribe who arrived in early December, said she left her village in Yei with her five children shortly after her husband was abducted by government soldiers.

"The government people, when they come, they speak in Dinka. If you don't respond to them they kill. If you're Dinka then you survive," Poni said. She has no idea of her husband's fate.

So far, Uganda is keeping its doors open to the humanitarian disaster, and sticking by its policy of granting refugees freedom of movement and work, and access to public services such as schools and clinics. But it says the pressures are huge.

"Uganda is receptive to refugees not because of adhering to its international obligations but because of our ideological pan-African stand," Information Minister Frank Tumwebaze said. "But the threats they pose are glaring."

(Writing by Ed Cropley; Editing by Philippa Fletcher)