Nearly a quarter of a billion people escaped slums in the past decade, but the housing effort was outstripped by population growth and rural exodus to the cities, the UN said.
A total of 227 million people rose out of slum conditions from 2000 to 2010, thanks especially to hard work in China and India, according to the UN Human Settlements Programme, also called UN-Habitat.
It means that the UN has already scored a rare success in its Millennium Development Goals. Under MDG 7, Target 11, UN members pledged to "achieve significant improvement" in the lives of at least 100 million slum dwellers by 2020.
The bad news is that from 2000-2010, the absolute numbers of slum dwellers increased from 776.7 million to 827.6 million.
"Cities are growing faster than the slum improvement rate," said Gora Mboup, a Senegalese who co-authored the report, State of the World Cities 2010/11: Bridging the Urban Divide. Half of the increase of 55 million extra slum dwellers came from population growth in existing slum homes; a quarter by rural flight to the cities; and a quarter by people living on the edge of cities whose homes became engulfed by urban expansion.
UN-Habitat warned: "Short of drastic action, the world slum population will probably grow by six million each year, or another 61 million people, to hit a total of 889 million by 2020."
UN-Habitat highlights
• Sub-Saharan Africa has the largest slum population, totalling 199.5 million people, or 61.7 percent of its urban population. It is followed by South Asia (190.7 million people, 35 percent of urban population) and East Asia (189.6 million, 28.2 per cent).
• China and India are lauded for making "giant strides" to improve the life of slum dwellers. China made improvements to the daily conditions of 65.3 million urban residents without shelter. The proportion of urban Chinese living in slums fell from 37.3 per cent in 2000 to 28.2 per cent in 2010. India, meanwhile, lifted 59.7 million out of slum conditions last decade. Slum prevalence now stands at 28.1 per cent.
• The world's three most "unequal cities" in terms of disparity of wealth among its inhabitants are all in South Africa: Buffalo City, Johannesburg and Ekurhuleni. The "most equal" cities are Chittagong and Dhaka in Bangladesh, which are also blighted by poverty.
• Urban sprawl, once associated only with cities in North America, is fast engulfing many developing countries as property developers promote life in the spacious suburbs. Sprawl causes transport problems because of the usual over-reliance on cars, can pose a threat to the environment if housing encroaches on sensisitive zones and also adds to social segregation, says the report.
• More than half of the world's population - 3.49 billion people, or 50.6 per cent of the total - now live in urban areas, it notes.
• Investigators determined that housing was a slum if it lacked at least one of out of these five amenities: it had a permanent structure; had less than three people sharing a room; access to water that was sufficient, affordable and could be obtained without extreme effort; a private toilet or a public one shared with a reasonable number of people; and secure tenure.