Ethiopia has been accused of deliberately underestimating the scale of a deadly drought facing millions of its people, some of whom are being deprived of emergency food aid by the country’s military.

The humanitarian crisis, caused by three years of failed rains, currently affects about 4.6 million people, though the official number could jump to as high as 6.7 million this week.

United Nations agencies say that the real number at risk is above 8 million, an estimate disputed hotly by Addis Ababa, which is insisting on publishing a much lower figure.

“The figure has risen very substantially, maybe even doubled,” said Sir John Holmes, the UN’s emergency relief co-ordinator, who visited Ethiopia earlier this month. “Any government doesn’t want to be perceived as always in the position of receiving aid.”

The crisis is at its most worrying in the vast deserts of the Ogaden region, where the UN’s World Food Programme (WFP) says in a confidential alert to donors that it is receiving “increasing reports of hunger-related mortality”. About two million people are at risk until the main rains fall next spring – if they fall at all. The Ogaden is Ethiopia’s biggest and most remote state.

Nomadic tribes there are resorting to eating dead leaves and cactus fruit to survive the worst drought since the famines of 1984-85, when an estimated one million Ethiopians died.

A twenty-mile trek on foot into the bush revealed mediaeval mud-hut villages, where ethnic Somali herdsmen say that their children have died after eating poisonous buds from trees, for lack of anything else to eat. Others say that they depend on camel milk and meat because cattle, sheep and goats have perished in their thousands.

“I am ill and hungry,” said one man, removing his shirt to reveal his rib cage visible through taut skin. “Because of the drought we have nothing to eat. The only people who receive food are the military forces.”

The UN has raised about 60 per cent of $325 million (£181 million) it is seeking in emergency relief for Ethiopia and says that it is suffering a shortfall of about 300,000 tonnes of aid.

The WFP has told donors that it blames Ethiopia’s “delays in recognising the extent of need” for causing the rapid depletion of existing food stocks. But a Channel 4 News investigation tonight claims that the army has withheld food from villages in the Ogaden deliberately as part of a “scorched earth” policy against separatist rebels of the Ogaden National Liberation Front (ONLF).

Herdsmen in villages almost completely cut off from the outside world said that many of their animals had been killed by Ethiopian soldiers, who also deprived them of water.

“We walk for eight hours to collect water,” said Abdi, a villager about three hours from Jijiga, the regional capital. “Then the military take the water from us. They say the rebels pass through our villages and that we give them supplies. But what can we give? We are dying of hunger. We have nothing to give to our own children.”

The UN says that it has negotiated with the Ethiopian army for the military’s role in food distribution to be kept to a minimum. “If there is a situation where food is taken by the military, we protest,” said Mohammed Diab, the WFP’s Ethiopia director.

However, a confidential investigation by USAid, the US Government’s disaster relief agency, complained in March that “literally hundreds of areas . . . have neither been assessed nor received any food assistance”, with “populations we met terrorised by the inability to access food”.

“This situation would be shameful in any other country,” the report concludes. “The US Government cannot in good conscience allow the food operation to continue in its current manifestation.” The US is spending more than £230 million on food aid for Ethiopia this year but is hamstrung from being too critical in public; Washington sees Meles Zenawi, Ethiopia’s Prime Minister, as an ally in the War on Terror after Ethiopia’s invasion of Somalia in 2005, which ousted an Islamist administration from power.

Britain has doubled its annual aid to Ethiopia in the last three years to £130 million, including £15 million this summer through the UN’s Humanitarian Response Fund, while Save the Children (SCF) is halfway through a campaign to raise £10 million for the country. Two SCF workers were expelled from the Ogaden last year amid allegations – rejected by SCF – that they had diverted food to ONLF rebels. The British charity abandoned a full-scale feeding programme, fearing supplies could be diverted.

Jonathan Rugman’s report on Ethiopia’s drought is on tonight’s Channel 4 News at 7pm.

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