The United Nations is warning that the number of people living in slums around the world could double to two billion within 30 years, unless there is immediate action to reverse the trend.

A report by the U.N. Human Settlements Program, HABITAT, says the worst affected areas now are in Asia with more than 550 million slum dwellers and Africa with nearly 190 million.

In total, the U.N. report estimates almost one billion people currently live in overcrowded areas which lack basic services. It predicts that number could double within 30 years.

The U.N. agency called on governments around the world to recognize the scale and seriousness of the looming crisis.

HABITAT's executive director, Anna Tibaijuka, says high poverty levels could affect global security. She said the poor are not terrorists, but that extreme deprivation could lead to anti-social behavior.

The U.N. report says there is little or no planning to accommodate the mass of people pouring into cities to seek a better life.

The report says the developed world also faces the problems of poverty with more than 50 million slum dwellers.