The world's population will hit 7.2 billion next month and 10.9 billion by 2100, with most of the growth a result of high birthrates in the developing world, the United Nations has said.

The UN's latest "World Population Prospects" report on Thursday said the number of people inhabiting the planet at the start of the next century could top 16.6 billion, or depending on the statistical model, could be as low as 6.8 billion.

In either case, the population in the world's poorest regions is anticipated to rise dramatically, the UN said.

The number of inhabitants in the world's least developed countries is projected to double, from 898 million inhabitants this year to 1.8 billion in 2050. The number will soar to 2.9 billion by 2100, the UN report said.

"Although population growth has slowed for the world as a whole, this report reminds us that some developing countries, especially in Africa, are still growing rapidly," Wu Hongbo, United Nations Undersecretary General for Economic and Social Affairs, said in a statement.

By contrast, population in the world's developed nations is expected to remain largely unchanged, inching upward from 1.25 billion this year to around 1.28 billion in 2100.

India rising

India's population is expected to surpass China's around 2028 when both countries will have populations of around 1.45 billion, according to the report. While India's population is forecast to grow to around 1.6 billion and then slowly decline to 1.5 billion in 2100, China's is expected to start decreasing after 2030, possibly falling to 1.1 billion in 2100, it said.

The report found global fertility rates are falling rapidly, though not nearly fast enough to avoid a significant population jump over the next decades. In fact, the UN revised its population projection upward since its last report two years ago, mostly due to higher fertility projections in the countries with the most children per women. The previous projection had the global population reaching 9.3 billion people in 2050.

"As a result, these populations are ageing rapidly and face challenges in providing care and support to their growing ranks of older persons," said John Wilmoth, director of the Population Division in the UN's Department of Economic and Social Affairs.

Wilmoth cautioned that "there is a great deal of uncertainty about population trends." He said projections could change based on the trajectories of three major components - fertility, mortality and migration.

Still, population growth until 2050 is all but inevitable.