The UN health agency on Wednesday launched a global '1,000 cities' drive to counter a triple threat to health in fast growing urban areas that are now home to over half the world's population.

The World Health Organisation predicted that most population growth in coming decades will take place in overcrowded, polluted, and often impoverished cities that house a concentrated array of health problems faced by local societies.

"Urban health matters in critical ways for more and more people," said WHO Director General Margaret Chan, urging cities to place health concerns at the centre of their planning.

"Poor health, including mental health, is one of the most visible and measurable expressions of urban harm," she told a WHO meeting.

The world's urban population passed 3.0 billion in 2007, exceeding the rural population for the first time, according to the United Nations.

The WHO warned that urban areas condense a threefold burden: infectious diseases exacerbated by poverty, noncommunicable diseases such as heart trouble, cancers and diabetes fuelled by smoking, unhealthy "convenient" diets and sedentary lifestyles, as well as injuries caused by accidents or crime.

The year-long campaign starting on World Health Day aims to mobilise 1,000 cities to open up public spaces to health, by closing off portions of streets to traffic, to encourage exercise in parks and clean up campaigns.

One of the WHO officials behind the drive, Lori Sloate, said it was important to forge a global network that would influence city planning and management, "while there's still time because we've just passed the tipping point."

By 2030, six out of 10 people will be city dwellers, rising to seven out of 10 people by 2050.

Big cities are also growing far more quickly than in the past, outpacing the ability of authorities to build or plan for essential infrastructure including adequate health services, water and sanitation, the WHO said in a report.

Living and working conditions vary widely both within and between cities across the world and are the "causes of the causes" of ill-health, according to the health agency.