GENEVA (AFP) -- By 2020, if nothing is done, 1.4 billion people -- the equivalent of the population of China -- will be living in slums and shantytowns, a specialist United Nations agency reported.

The UN human settlements program (UN-Habitat) said in a report that governments should not try to stem the flow of people to the cities but manage it instead.

At the moment around one billion people live in slums, or almost one urban resident in three, the agency says in its 2006/7 report on the world's cities. Their population is growing by 2.2 percent a year, with a rate of 4.5 percent in sub-Saharan Africa.

But the pace of growth is increasing, and by 2020 slum populations will swell by 27 million a year, compared with 18 million between 1990 and 2001.

At the beginning of next year the number of urban dwellers will for the first time equal the rural population. That has been the case in Europe since the end of World War II but will not be the case in Africa and Asia until 2020.

Developing countries are catching up fast, with 95 percent of the growth in urban populations taking place in the south, so that by 2030 out of 8.1 billion people living on the planet five billion will live in conurbations. UN-Habitat wants to cut the number of slum dwellers to around 700 million by 2020.

"Economic growth doesn't lead automatically to slum reduction," said Eduardo Moreno, one of the report's authors, who called for positive policies to improve the urban environment.

North African states, in particular Egypt, took such steps some 10 to 15 years ago and the number of slum dwellers in the region has begun to fall, Moreno told reporters.

Some governments have improved living conditions by providing water, drains, electricity or even technical help for building.

But if slums have sprung up in areas at risk, as for example zones liable to flooding, they have no choice but to rehouse the inhabitants, the agency says.

Contrary to popular wisdom, life in disadvantaged urban areas is not necessarily better than in the countryside, said UN-Habitat, basing its conclusion on a number of human development indices.

In poor countries 40 percent of children living in slums are undernourished, the same proportion as in the countryside. Infantile mortality rates are comparable and HIV-AIDS infections and diarrhea-related illnesses worse in slums. But there can be no question of governments forcing slum dwellers back to the countryside, said UN-Habitat spokesman Sharad Shankardass.

"Cities are places of opportunities. It is the first step moving out of poverty for many people coming from the countryside, said Nefise Bazoglu who edited the report. "Governments should accept urbanization, it cannot be reversed or curbed," she said. "A well-managed urbanization is a good engine for development."