India and China will witness the largest growth in their urban populations in the next four decades, posing new challenges of providing jobs, housing, energy and infrastructure to their people, the United Nations said on Thursday.

The UN's '2011 Revision of the World Urbanisation Prospects' said Africa and Asia will lead the global urban population growth in the next four decades with the largest increases expected in India, China, Nigeria, US and Indonesia.

India will add another 497 million to its urban population between 2010 to 2050, while China will see 341 million people moving into urban cities followed by Nigeria (200 million), the US (103 million) and Indonesia (92 million).

The projected increase in urban population in India and Nigeria between 2010 and 2050 will be higher than that of the past 40 years. This trend is particularly prominent in Nigeria, where its urban population grew by only 65 million between 1970 and 2010, but is projected to increase by 200 million between 2010 and 2050 – the third largest increase in urban population of all countries of the world.

Africa's urban population will increase from 414 million to over 1.2 billion by 2050 while that of Asia will soar from 1.9 billion to 3.3 billion.

The UN said both regions would together account for 86 per cent of all increase in the world's urban population. This ‘unprecedented’ increase in urban population will provide new opportunities to improve education and public services in Africa and Asia. However the UN warned that the trend would also ‘pose new challenges of providing urban jobs, housing, energy and infrastructure to mitigate urban poverty, expansion of slums and deterioration of the urban environment’.

The UN said with half of humanity living in cities today, urban areas are where pressures of migration, globalisation, economic development, social inequality, environmental pollution and climate change are most directly felt.

Cities are, at the same time, engines of the world economy and centres of innovation where many solutions to global problems are being piloted, it said.

The projections assume significance as nations prepare to meet in Brazil for Rio+20 summit - the United Nations Conference on Sustainable Development. Rio+20 Secretary-General Sha Zukang said world leaders, along with thousands of participants from the private sector and civil society, will need to use the UN conference in June to come up with ‘concrete action plans to realise sustainable cities for the future we want’.

The Rio conference would help shape ideas on how the world can reduce poverty, advance social equity and ensure environmental protection on an ever more crowded planet, Zukang said.

According to the UN estimates, among 450 urban areas with one million or more inhabitants in 2011 (representing 1.4 billion people), 60 per cent, or about 890 million people, are located in regions exposed to at least one major type of natural disaster risk.

Cities in Europe and Africa are the least exposed overall, with only 26 per cent and 37 per cent, respectively, of their cities with one million inhabitants or more living in regions exposed to at least one major type of natural disaster risk.

However, cities in Latin America and the Caribbean, and especially in Asia are often located in regions exposed to natural hazards.