About 1.4 billion people worldwide will be living in slums by 2020 unless action is taken to improve conditions for the urban poor, according to a U.N. report released Friday.

The number of slum-dwellers globally is expected to grow from the current 1 billion — nearly all in the developing world — as city populations swiftly rise, the U.N. Human Settlement Program said.

"Slums in many cities are no longer just marginalized neighborhoods housing a relatively small proportion of the urban population," the report said. "In many cities, they are the dominant type of human settlement."

The global slum population is expected to grow by 27 million every year until 2020, the agency said.

Authorities have traditionally viewed slums as temporary settlements that would disappear as cities developed and incomes rose. But the report said slums — lacking durable housing, sufficient space and easy access to safe water or adequate sanitation — are continuing to grow and becoming a permanent feature of many cities, particularly in sub-Saharan Africa and Southeast Asia.

"There are two cities within one city — one part of the urban population that has all the benefits of urban living, and other part, the slums and squatter settlements, where the poor often live under worse conditions than their rural relatives," said Anna Tibaijuka, chief of the U.N. program.

The report said the only part of the developing world to have limited slum growth is North Africa, where countries such as Egypt and Tunisia have made large investments in improving living conditions, as well as trying to dissuade people from moving to the slums.

As of last year, just under half of the world's population of 6.45 billion lived in cities. The urban population is forecast to reach half the world's population in 2007, when the overall figure is predicted to be 6.57 billion.

Trends indicate the number of urban dwellers will reach 5 billion by 2030, out of a total population of 8.1 billion.