A UN report says the world population will reach 9.8 billion in 2050, with India overtaking China as the most populous country in the world.

India is one of the countries with the highest population growths in the world and its population is set to overpass China's in 2024, according to the report by the Department of Economic and Social Affairs' Population Division.

Currently, India's 1.34 billion population and China's 1.41 billion together constitute more than a third (37 percent) of the world population of nearly 7.6 billion.

In a previous 2015 projection, the UN had estimated 2022 to be the year India reaches the No.1 place.

Nigeria, meanwhile, has the highest population growth among the most populous countries of the world.

By 2050, half the world's population growth will be concentrated in just nine countries — India, Nigeria, Congo, Pakistan, Ethiopia, Tanzania, United States, Uganda and Indonesia, the UN agency forecasts.

The report stated the 10 most populous countries with low fertility levels are China, United States, Brazil, Russia, Japan, Vietnam, Germany, Iran, Thailand and United Kingdom.

It said 83 million people are added to the world population each year.

"With roughly 83 million people being added to the world's population every year, the upward trend in population size is expected to continue, even assuming that fertility levels will continue to decline," the report's authors said.

The fertility rate in recent years has been declining in almost all areas across the globe, particularly in Europe. A quarter of Europe's population is already aged 60 or over, according to the report.

The number of persons aged 60 or above is expected to more than double by 2050 and more than triple by 2100, it added. The aging population is projected to rise from 962 million globally in 2017 to 2.1 billion in 2050 and 3.1 billion in 2100.

"Europe has the lowest fertility level, estimated at 1.6 births per woman in the most recent period," said John Wilmoth, director of the Population Division.

 Africa has the highest fertility, with around 4.7 births per woman. The populations of 26 African countries are expected to at least double.

"The population in Africa is notable for its rapid rate of growth, and it is anticipated that over half of global population growth between now and 2050 will take place in that region," Wilmoth said. 

Wilmoth said the report reviewed information on populations from 233 countries or areas of the world.