Tension had been simmering for weeks after members of the Gabra tribe crossed into Ethiopia in search of pastures for their camels, cattle and goats.

The Boran tribesmen, waving AK47 assault rifles, separated the Gabra men from their women, children and animals, before forcing them to lie on the ground — but not before two men had been shot dead in the confusion. They warned the Kenyans to stay out of Ethiopia and said they were confiscating their animals, before disappearing back into the night.

The rustlers escaped on foot with more than 2,000 animals and forced 1,000 people from their homes, according to aid workers in the area.

That attack, a week ago, was the first in a series of clashes across the remote badlands of northern Kenya that have left 13 dead during the past week. Another 10,000 people have been forced to flee as warring tribes launched cattle raids and counter-attacks.

Aid agencies are delivering food, water and tarpaulins to families seeking refuge from violence in the Marsabit and Samburu regions of Kenya.

Abbas Gullet, secretary-general of the Kenya Red Cross Society, said: “People are running for their lives, camping out in church and school grounds — wherever they can find shelter — and these people need urgent assistance.”

Rustling is an age-old practice among the closely related tribes that eke out a precarious existence in a bleak, unforgiving part of Kenya that has been largely forgotten by the Government in Nairobi.

In an area beyond the reach of mobile phones and with few roads, let alone banks, animals are counted as a form of wealth. Stealing them from rival communities is seen as a rite of passage for young warriors.

In recent years the raids have taken on a more violent nature, fuelled by an influx of guns from civil wars in Southern Sudan, Uganda and Somalia. This week’s clashes erupted as much-needed rain fell on the country’s parched soil. A regional drought turned pastureland and waterholes to dust this year, sending 11 million people to the brink of famine across the Horn of Africa.

Nomadic herders have been particularly badly hit. Oxfam estimates that up to 80 per cent of domestic animals have succumbed to hunger or thirst.

The herders have had to look further afield for pasture and water as they try to save their remaining cattle, goats and camels. But the movements have brought rivals into conflict as, for example, Kenya’s northern Gabra crossed into Ethiopia on to land coveted by the closely related Boran.

Further south, trouble has erupted in the Samburu district, where Pokot tribesmen killed five people and stole nearly 1,500 cattle from the Samburu tribe. Two Pokots were killed in a revenge attack, according to police.

Mr Gullet said that the latest conflict marked an escalation after years of low-level attacks.

“Now after such a bad drought and with the rains finally arriving, herders who lost animals are taking the opportunity to restock. In some ways this has become a question of survival rather than the usual raids, which are a chance to increase your wealth,” he said.

“We are also seeing something new, in that this used to be all about the animals and it was almost a taboo to kill people. That has changed.”

The outbreak of violence was not unexpected. Wangari Maathai, who won the 2004 Nobel Peace Prize, has long given warning that scarce water and grazing would increase the number of tribal flashpoints. Last year there was a massacre last year in which 70 Kenyans, including 22 children, were killed by warriors from Ethiopia.

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