The German development minister slammed the European Union for failing to control the crisis, saying the border-free Schengen zone has "collapsed".

He also claimed the number of people living in Africa would "double in the coming decades", with Nigeria's population swelling from 186 million to 400 million.

Last year Chancellor Angela Merkel opened Germany's doors to an unlimited number of Syrian refugees.

But she has since called for stricter laws regulating asylum seekers following a string of alleged sex attacks by gangs of migrants in Cologne.