African cities need to move toward greener water and sanitation projects, such as rainwater collection, to keep pace with booming urban populations, a new United Nations report said Monday.

"We need to turn to new and innovative engineering solutions and those that in particular address more green infrastructure," Tim Kasten of the United Nations Environment Programme (UNEP) told journalists on the sidelines of a summit for World Water Day 2011.

Traditional engineering solutions were not able to keep up with swelling cities, says the report, "Green Hills, Blue Cities", produced by UNEP and UN Habitat.

It argues that access to safe drinking water and sanitation has not matched massive population growth.

"We're not able to keep pace. Some of the solutions of the past are not necessarily going to be able to take us into the future," said Kasten.

"Africa is urbanising at a very rapid pace, from 205 million people living in cities in 1990 to today where we now stand at 400 million in 2011. And by the year 2050 we will have over 1.2 billion people living in urban centres in Africa."

The report's recommendations include collecting rainwater and preserving natural systems such as forests and wetlands, which could help filter wastewater that goes around 90 percent untreated in Africa.