The UN in its biannual population report on Monday also warned about a growing aging population in Europe and North America, the Efe news reported.

The report though stressed that the population would grow at a slower pace than it had been forecast two years back.

The countries that will experience the greatest population growth, in descending order, are predicted to be India, Nigeria, Pakistan, the Democratic Republic of the Congo, Ethiopia, Tanzania, Indonesia, Egypt and the US.

In the updated document, "The World Population Prospects 2019: Highlights" the international body has stressed that in the next 30 years the world's current population of 7.7 billion will grow by about 2 billion and the aging of the world's population will also spike "due to increasing life expectancy and falling fertility levels"

In its 2017 study on population growth, the UN had forecast that there would be 9.8 billion people on the planet in 2050 and 11.2 billion in 2100.

Along those lines, the report emphasises that in 2050 about 16 per cent of the world's population will be over 65 years of age, compared with 9 per cent at present. By regions, Europe and North America will have a much higher than average number of over-65 citizens: 25 per cent.

The UN warns that this aging trend -- in 2050 will be 426 million people over age 80, compared to 143 million now -- bringing with it a decline in the proportion of the working age population.

It will hence exert pressure on social security systems that rely on wage-earners to contribute part of their pay to such safety-net programs.

The algorithm used to forecast population growth also found that the number of countries experiencing a reduction in their populations is increasing.

The report notes that since 2010, a total of 27 countries or areas have experienced a decline of 1 per cent or more in the size of their populations due to lower fertility levels. With the fertility rate having fallen from 3.2 births per woman in 1990 to 2.5 in 2019, it is now predicted to go down to around 2.2 by 2050.

The UN calculates that, without migration, a level of 2.1 births per woman is necessary to merely ensure generational replacement of the population. Anything less than that leads to overall population decline and levels higher than that result in a growing population.

Between 2019 and 2050, the study estimates 55 of the world's present-day countries to experience the 1 per cent or greater decline in their population. In 26 of these countries the reduction could also touch 10 per cent.

The report also focusses on out-migration as a key component of population change in countries like Bangladesh, Nepal and The Philippines -- from where migrant workers are in demand.

Besides, sometimes out-migration could also be caused by violence, lack of security or wars -- as in Myanmar, Syria and Venezuela.

In contrast, the report says that between 2010 and 2020 about 10 countries -- including Belarus, Estonia, Germany, Hungary, Italy, Japan, Russia, Serbia and Ukraine -- will experience a net inflow of migrants, which will help compensate their population loss due to deaths that exceed births.