A new report says Africa and Asia will see the fastest urban population growth in the next four decades.

A United Nations study issued Thursday says Africa's urban population will increase almost three times, from 414 million to more 1.2 billion by 2050, and that's Asia's will grow from just under 2 billion to 3.3 billion.

In the report, the U.N. says India, followed by China and Nigeria will see the biggest increases.

The United States and Indonesia will also experience significant population increases in urban areas by 2050.

The report says such rapid urban population growth could open up new opportunities in education for Africans and Asians, but will also pose challenges in housing, employment, energy and the urban environment.

According to the report, earthquakes and floods also pose a danger to densely populated urban centers with more than 1 million people. It says Asia is the most exposed region for such climactic events, while Europe and Africa are the least exposed overall.