World population is set to rise 33 percent from 7.3 billion today to 9.7 billion in 2050, despite a slowing down in annual growth, says the latest UN projection.

There is also a very high probability that the numbers could go up to 10 billion by 2050 and touch 12.5 billion in 2100.

"Continued population growth until 2050 is almost inevitable, even if the decline of fertility accelerates," says the report, World Population Prospects: the 2015 revision.

Only Europe will see a decline in numbers from 738 million now to 646 million at the turn of the century.

Nine countries that will contribute half the world's population growth between now and 2050 are: India, Nigeria, Pakistan, Democratic Republic of Congo, Ethiopia, Tanzania, US, Indonesia and Uganda.

Africa will drive more than half of the world's population growth with the population in 28 of the continent's countries doubling in 35 years. By 2050, Nigeria's population will surpass that of the US.

Africa will be the only area experiencing population growth after 2050.

Most of the 21 "high-fertility" countries, where the average woman has five or more children over her lifetime are in Africa. Among the largest in this group are Nigeria, Democratic Republic of Congo, Tanzania, Uganda and Afghanistan.

India will overtake China as the world's most populous country with its population touching 1.5 bn in 2030.

China's population stands at 1.4 bn and India's at 1.3 bn, representing 19% and 18% of the world's population, respectively. China is set to experience a slight decrease after the 2030s.

Despite a slowdown from 1.24% annual growth to the present 1.18%, owing largely to fertility rates dropping, world population is set to grow in countries where populations are already large or where high numbers of children are born.

"Regardless of the uncertainty surrounding future trends in fertility in Africa, the large number of young people currently on the continent who will reach adulthood in the coming years and have children of their own, ensures that the region will play a central role in shaping the size and distribution of the world's population over the coming decades," says the report.

Countries of Europe and northern America, 20 Asian countries, 17 Latin American or Caribbean ones, three in Oceania and one in Africa have low levels of fertility with women having fewer than two children on average.