ROME - The United Nations Food and Agriculture Organization (FAO) on Thursday urged policymakers to ensure that African cities will be "green" enough to meet their nutrition and income needs in a sustainable way.

In its first status report on African urban horticulture titled "Growing greener cities in Africa," FAO warned that the continent's urban population is growing faster than that of any other region, but many of its cities are not keeping pace with the increasing demand for food.

More than half of all urban Africans live in slums, up to 200 million survive on less than $2 a day, and poor urban children are as likely to be chronically malnourished as poor rural children, the Rome-based organization said.

FAO urged national and local governments to work together with growers, processors, suppliers and vendors to give market gardens and urban agriculture - the most important source of fresh produce - the support necessary for sustainable development.

"African policymakers need to act now to steer urbanization from its current, unsustainable path towards healthy, greener cities that ensure food and nutrition security, decent work and income, and a clean environment for all their citizens," it said.

FAO's Program for Urban and Peri-urban Horticulture will contribute to help cities ensure a year-round supply of affordable, fresh food.

By 2020, 24 of the world's 30 fastest growing cities will be African, while between 2010 and 2030 the urban population of sub-Saharan Africa is expected to double from about 300 to 600 million.

The report was released ahead of the sixth session of the World Urban Forum which will be held in Naples, southern Italy, on Sept 1-7.