The world population is estimated to surpass 9 billion persons by 2050, according to a report released Tuesday by the United Nations' Population Division.

According to the report, the world population will likely increase by 2.5 billion over the next 43 years, passing from the current 6.7 billion to 9.2 billion in 2050.

This increase is equivalent to the total size of the world population in 1950, and it will be absorbed mostly by the less developed regions, whose population is projected to rise from 5.4 billion in 2007 to 7.9 billion in 2050.

In contrast, the population of the more developed regions is expected to remain largely unchanged at 1.2 billion, and would have declined, were it not for the projected net migration from developing to developed countries, which is expected to average 2.3 million persons annually.

As a result of declining fertility and increasing longevity, the populations of more and more countries are aging rapidly. Between 2005 and 2050, half of the increase in the world population will be accounted for by a rise in the population aged 60 years or over, whereas the number of children (persons under age 15) will decline slightly.

The report predicts that the number of persons over 60 will hit1 billion by 2050.

The projected population trends also depend on achieving a major increase in the proportion of AIDS patients who get anti-retroviral therapy to treat the disease and on the success of efforts to control the further spread of HIV.

"We are expecting a relatively good coverage of anti-retroviral drugs in 31 of the most affected countries by 2015," Hania Zlotnik, director of the UN Population Division, told a press briefing in New York held to launch the report.

"According to our estimates, 70 percent of the affected people are going to get treatment. Given that, we're postponing the deaths by several years," she said. On average, those receiving treatment are expected to live 7.5 years longer than those who are not.