Once again, African children are dying of hunger. But why is famine afflicting places of such natural wealth?

Hunger stalks southern Africa. In Malawi, the men risk their lives to dive for water-lily bulbs in crocodile-infested rivers and kill mice to eat and sell. The women gather wild grasses and boil up weeds to feed their children.

Nearly 13 million people are on the brink of starvation as the worst food crisis in a decade spreads across six countries: Malawi, Mozambique, Swaziland, Zambia, Zimbabwe and Lesotho.

A meeting in Johannesburg of aid officials and government leaders has heard a stark warning from the United Nations. Jean-Jacques Graisse, the deputy director of the UN World Food Programme (WFP), said: "This is a crisis of enormous dimensions. The situation worsens each day."

More than 1 million tonnes of emergency food aid is required to avert an unfolding disaster in which many people will die. "We'll talk about a famine the day we see thousands and thousands of people dying," said Mr Graisse.

"At this stage, we have a great number of people facing malnourishment and starvation in some areas, and if we do nothing, we will face a famine."

The most vulnerable – the very young and the sick – are already perishing, and millions are surviving on staple foods supplemented by scraps foraged from the wild.

The British charity Christian Aid is assessing the crisis in Malawi and is poised to launch a major appeal. "People here are so desperate they are hunting mice to eat and sell," said the agency's Dominic Nutt, speaking from Blantyre.

"Many villages are getting no help at all. We saw the fresh graves of hundreds of people in one village who have died since February. Children, many of them HIV orphans, have swollen stomachs from chronic malnutrition," he said.

In Zambia there has been a 90 per cent crop failure and the price of maize has spiralled Joanne Muté, of the aid agency Tearfund, who is in the worst-hit area in the south of the country, said: "Families here are eating roots and even rats when they can find them."

Yesterday, the UN organisations released new estimates of food-aid needs in the six countries, based on the reports of its own fact-finding missions.

The total number of people going hungry is 7.7 million, and they urgently require 262,000 tonnes of food aid. This number will soar to 11.5 million people from September to November, and to 12.8 million from December to March. Some four million tonnes of aid will be required to see the region through to the end of the year, the UN said.

By then, there will be 6 million Zimbabweans in need of emergency food aid, 3.2 million people in Malawi, 2.3 million in Zambia, 500,000 in Mozambique, 445,000 in Lesotho and 231,000 in Swaziland.

Southern Africa is a region of enormous natural wealth. So why are 13 million people enduring these pitiful conditions?

Drought and floods are certainly to blame, but man-made calamities, such as the decision of the authorities in Malawi to sell off a strategic 167,000-tonne grain reserve and President Robert Mugabe's controversial land seizures in Zimbabwe, have played a big part.

It is no coincidence that the countries worst hit by food shortages are those in the greatest political turmoil – Zimbabwe, Malawi and Zambia. But only now are governments in the region admitting that they share the blame.

Non-governmental organisations have criticised the IMF for encouraging Malawi, where up to 3.2 million people are going hungry, to reduce its grain reserves. Yet Malawi itself ordered the total sale, and the government is now investigating where the money went. The privatisation of agencies that previously regulated the price paid to farmers and the distribution of seed and fertiliser has also contributed to the crisis.

Zimbabwe used to export food to the entire region. Now an extraordinary 5.3 million people need aid.

Yesterday a minister in Mr Mugabe's government admitted for the first time that politically inspired invasions of thousands of white-owned commercial farms were partly to blame for the crisis.

Simba Makoni, Zimbabwe's Finance minister, conceded at a summit in Durban that his country's land-redistribution policies had reduced the amount of commercial farmland growing maize, the staple food. "It compounds, it exacerbates, but it is not the primary cause of the problem," he added, placing the blame primarily on drought.

The ruling Zanu-PF party is also accused of delivering food aid only to its supporters.

Nobody at the gleaming International Conference Centre in Durban is going hungry. There, in a separate meeting, eight African presidents and 700 delegates from 47 countries (28 of them African) are thrashing out ways of implementing Nepad – the New Economic Partnership for African Development – the continent's renaissance plan, and attracting funding for it at the upcoming G8 summit in Canada.

The summit in Durban and the famine gathering in Johannesburg are inextricably linked. Nepad would help Africa cope with famine in the future by tackling issues such as the need to stamp out corruption.

The meeting in Johannesburg, organised by the WFP and the Food and Agricultural Organisation, has brought together some 100 aid agency, donor and government officials to coordinate their responses and to tackle issues such as accountability and donor concerns about giving aid to governments they view as unreliable. The focus is also on an Aids epidemic that is deepening the effects of crop failure.

While climate has played an important role in the impending food security crisis the need for more effective governance is highlighted by a new problem being faced this year, according to Brenda Barton, of the WFP.

"Governments said they would import large amounts of food, but these have largely not materialised," she said. "Lack of imports has meant lack of food on the market, which has fuelled price hikes. This has made the poor unable to buy food when their harvests run dry." Zimbabwe, like Malawi, sold maize reserves even as shortages escalated.

Zimbabwe has had its longest dry spell in 20 years. But that is far from the only reason for its worst agricultural crisis since the country achieved independence in 1980.

Drought has been aggravated by a sharp fall in maize production by commercial farmers, who used to produce a third of total cereals but whose farming has been severely disrupted by land reform and invasions.

While the area of planted maize has grown through the resettlement of peasant farmers, the area of commercial farmland planted with maize – the most productive sector – has plummeted by 62 per cent in two years since the start of farm occupations. Maize production is 77 per cent less than it was two years ago, other cereal production is 67 per cent less.

The WFP says: "The extremely poor main season has been caused by a severe drought between January and April in most parts of the country and the near collapse of large-scale commercial production due to land reform activities. Zimbabwe is facing an immediate, serious food crisis. Unless sufficient food can be imported, and the poorest people can access it, severe malnutrition and death caused by hunger will occur in the coming months".

Mr Mugabe may also be facing a torrid time from fellow African political leaders in the coming month, as they prepare to sell the Nepad scheme to the G8. Zimbabwe's political crisis could make it a test case for the regeneration plan, which promises political and economic reform in return for debt relief and increased Western aid.

Zimbabwe has expressed fears that it will be "abandoned" by the rest of Africa. Olusegun Obasanjo, the Nigerian President, said that Africa would not hesitate to "sanction itself" if necessary.