LONDON, Feb 21 (Thomson Reuters Foundation) - The U.N. children's agency UNICEF said on Tuesday nearly 1.4 million children were at "imminent risk" of death in famines in Nigeria, Somalia, South Sudan and Yemen.
Famine was formally declared on Monday in parts of South Sudan, which has been mired in civil war since 2013.
People are already starving to death in all four countries, and the World Food Programme says more than 20 million lives are at risk in the next six months.
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The United Nations defines famine as when at least 20 percent of households in an area face extreme food shortages, acute malnutrition rates exceed 30 percent, and two or more people per 10,000 are dying per day.
Here are details about some of the major famines around the world in the last 100 years:
In 2011, Somalia suffered a famine that killed 260,000 people in south and central regions. The famine was declared in July, but most people had already died by May.
Years of drought, that have also affected Kenya and Ethiopia, have hit harvests and conflict has made it extremely difficult for agencies to operate and access communities in the south of the country.
U.N. declared Somali famine over in February 2012 following an exceptional harvest after good rains and food deliveries by aid agencies.
From 1995-1999 between 2.8 million and 3.5 million people died because of a combination of flooding and government policy in the reclusive state.
The Marxist policies of Mengistu Haile Mariam, which he began abandoning in 1990 with some economic reforms, left a country ravaged by economic decline, famine and regional conflicts that consumed half the state budget.
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In 1984-85, in the famine, up to one million Ethiopians starved to death.
For months in 1984, Mengistu denied the devastating famine in Ethiopia's north. Aid workers later recalled he flew in planes loaded with whisky to celebrate the anniversary of his revolution, as hunger deepened.
Bob Geldof, after watching pictures of the famine, organised Live Aid in 1985 to try to alleviate the hunger. Watched by 1.5 billion people, it raised $100 million for Africa's starving.
Up to 2 million died of famine following a decade of conflict, first during the 1970-1975 civil war, then during the brutal Khmer Rouge era until 1978 and finally in the aftermath of the Vietnamese invasion that ended Khmer Rouge rule in 1979.
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Between 10 and 30 million people died as a result of Mao Zedong's Great Leap Forward in the late 1950s.
His plan involved modernizing agriculture and increasing grain production however officials often exaggerated the size of harvests, and in many places the entire grain harvest was seized.
China's leaders appeared to have been unaware of the severity of the famine as from 1958 until 1961, China doubled its grain exports and cut imports of food.
Up to 8 million people died as a result of Josef Stalin's massive industrialisation programme in the Soviet Union in the early 1930s, during which the government seized grain for export. It needed the hard currency to buy industrial equipment.
When people in the Ukraine reported a famine, Stalin punished them by refusing to send them food aid.
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Sources: Reuters/Thomson Reuters Foundation/WFP/UN (Reporting by Magdalena Mis @magdalenamis1,; Editing by Ros Russell; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, property rights, climate change and resilience. Visit http://news.trust.org)