More than half a billion people across the world lack access to clean water, with most of them living outside urban centres, according to an international charity.

In a report released on Wednesday to mark World Water Day, WaterAid called for action to tackle existing challenges to water security and ensure reliable access to safe water.

"Today 663 million people globally are still without clean water and the vast majority of them - 522 million - live in rural areas," the report said.

OPINION: The politics of water insecurity

According to WaterAid, Papua New Guinea, Mozambique and Madagascar are among the countries worst off.

They are also among the 20 percent of nations most vulnerable to climate change and least ready to adapt.

The report said that the country with the greatest percentage of the rural population without access to clean water was Angola, at 71 percent.

The UN Environment Programme forecasts that water demand - for industry, energy and an extra billion people - will increase 50 percent by 2030.

"There is an absolute necessity to increase water security in order to overcome the challenges brought on by climate change and human influence," said Benedito Braga, head of the World Water Council, an umbrella grouping of governments, associations and research bodies.

In a new report published on Wednesday, the United Nations said that recycling the world's wastewater - runoff from agriculture, industry and expanding cities - would ease global water shortages while protecting the environment.

This is especially true in poor countries where very little, if any, wastewater is treated or recycled.

"Neglecting the opportunities arising from improved wastewater management is nothing less than unthinkable," said Irina Bokova, director-general of UNESCO.

High-income nations treat about 70 percent of the wastewater they generate, a figure that drops to 38 percent for upper middle-income countries.

In low-income nations, only 8 percent of industrial and municipal wastewater undergoes treatment of any kind.

Chemicals and nutrients from factories and farms create dead zones in rivers, lakes and coastal waters, and seep into aquifers.

More than 800,000 people die each year because of contaminated drinking water, and not being able to properly wash their hands.

Water-related diseases claim nearly 3.5 million lives annually in Africa, Asia and Latin America - more than the global death toll from Aids and car crashes combined.



Besides reducing pollution at the source, the World Water Development Report emphasised removing contaminants from wastewater flows, reusing water and recovering useful by-products.



"Up to now, decision-makers have mainly focused on supplying clean water rather than managing it after it has been used," said UNESCO’s Richard Connor, the UN report’s lead author.



"The two aspects are inextricably linked."



Water can be used over and over, he added, pointing to the fact that water from several major rivers in the United States is recycled up to 20 times before reaching the ocean.