An estimated 3.3 billion people will live in cities by next year, marking the first time in history that the majority of humans will live in urban centres.

A UN report released Wednesday predicts the biggest growth in Africa and Asia, where urban populations are expected to double between 2000 and 2030. By then, nearlyfive billion people worldwide will live in cities, according to the State of the World Population 2007 report.

But without access to housing or jobs, many of these new city dwellers will be poor, prompting urban poverty rates to outpace rural ones.

"In 2008, half of the world's population will be in urban areas, and we are not ready for them," UN Population Fund executive director Thoraya Ahmed Obaid told the Associated Press in London, where the report was released.

Identifying inadequate shelter as one of the main causes of poverty, the report found that policy-makers have tried to curb urban growth by discouraging migration and reducing low-income housing.

"Cities see poor people as a burden," Obaid said.

"Investing in them in terms of shelter, education and so on would mean you have a good economic force that can work and create even further economic growth coming from cities."

In an addendum to the report, Vancouver is highlighted as an example of the dangers of urban population growth without proper planning. Booming real estate prices are contrastedwith the city's dilapidated Downtown Eastside, where HIV rates match Botswana's at 30 per cent.

"This is the kind of price that a city — any city — will pay if it fails to support, plan for or house an expanding population of the urban poor," the addendum said.

Small to mid-sized cities of around 500,000 will bear the brunt of urban population growth, driven more by birth rates than rural migration. Toallay this increase, the report recommends reducing poverty, promoting gender equality and providing better access to reproductive health services.

If these cities fail to address the needs of their burgeoning populations, Obaid said, many could face social unrest and even religious extremism.

"Extremism is often a reaction to rapid and sudden change or to a feeling of exclusion and injustice, and the cities can be a basis for that if they are not well managed," Obaid said. "It's very much an urban phenomenon."