Fast population growth in small cities is "considerably increasing" the risk of catastrophes in the world, the United Nations warned Sunday.

Half of the world's urban population lives in cities of less than 500,000 people, the world body said in its Human Settlement Global Report 2007 published in Mexico City.

The report added that settlements of this size are proving to be the most hazardous places to live.

Government officials and society lack the capacity to face natural disasters in these "small" settlements, which have been growing fast and disorderly, the UN report said.

"The poorest, (along with the most) politically and socially isolated are always more vulnerable," it said, adding that disasters almost always occur in cities where construction codes are not respected.

"This situation, more than any other political challenge, underlines the need to relate social policies with technical and engineering solutions to avoid risks," the report said.

It indicated that the current level of city planning lags behind population growth, calling for planning techniques to be enhanced to meet future requirements.