Growing inequality in US cities could lead to widespread social unrest and increased mortality, says a new United Nations report on the urban environment.

"High levels of inequality can lead to negative social, economic and political consequences that have a destabilising effect on societies," said the report. "[They] create social and political fractures that can develop into social unrest and insecurity."

According to the annual State of the World's cities report from UN-Habitat, race is one of the most important factors determining levels of inequality in the US and Canada.

"It is clear that social tension comes from inequality. The trickle down theory [that wealth starts with the rich] has not delivered. Inequality is not good for anybody," said Anna Tibaijuka, head of UN-Habitat, in London yesterday.