A U.N, agency says humanity will have to undergo a ?revolution in thinking? to deal with a doubling of urban populations in Africa and Asia.

The U.N. Population Fund says the number of people in African and Asian cities will grow by 1.7 billion by the year 2030. And worldwide, the number of city dwellers will reach five billion or 60 percent of the world?s population.

The agency has released a new report: The State of the World Population 2007: Unleashing the Potential of Urban Growth. It says, ?Globally, all future population growth will take place in cities, nearly all of it in Africa, Asia and Latin America.? What?s more, the growth marks ? what the report calls ? ?a decisive shift from rural to urban growth, changing a balance that has lasted for millennia.?

Fama Ba, director of the UNFPA?s Africa Division, says this offers a challenge to urban planners and policy makers.

?Right now, we know that in the urban areas they?re already facing a lot of problems in terms of poverty, in terms of lack of access to basic social services like education and health, in terms of violence, in terms of crime and lack of safe drinking water [and] sanitation. And now, with the acceleration of urbanization, you can figure out what this will look like around 2020 or 2030,? she says.

Ba says despite the problems and challenges, it would be a mistake for African countries to try to prevent urbanization.

?Most of them have been trying to discourage urbanization, to discourage people moving from rural areas to urban areas. First thing is not to try to avoid that because this is inevitable and that the developing countries have to accept urbanization as something that is [a] potential ally,? she says.

The U.N. Population Fund official says urbanization can both trigger and encourage economic growth.

?When you are operating in an urban area with a concentration of population, you can provide services in a more effective and also [a] cheaper way because you have population concentrated, which would be different if you had the population scattered around different places where you?d need to bring the services. So, I think that?s an opportunity,? she says.

She says that urbanization would also provide more opportunities for women and young people, including employment and social services.

Ba says migration is not the driving force behind growing city populations.

?The increasing rate of urbanization is mostly due not to rural/urban migration, but rather to natural increase, population increase. Meaning that these are people who are being born in urban areas. Therefore they are young people,? says Ba.

The U.N. Population Fund report says providing shelter and opportunities to the poor should be a priority for city authorities and urban planners.

She says, ?Maybe one of the major challenges for policymakers is to involve the poor populations in the urban areas, developing policies, planning and developing programs. Because so far, all the programs that are set up, and the policies, do not take into account the needs of the poor population in the urban areas.?

Ba also says one of the Millennium Development Goals is to alleviate poverty.

?This fight against poverty will be waged in urban areas. And that whatever is happening there now will shape the future of the urban areas, the region and also even the world,? she says.

The U.N. Population Fund report says while mega-cities ? those with more than 10 million people ? will continue to grow, most people will live in cities with populations of 500,000 or less.