Stresses on water supplies aggravated by climate change are likely to cause more conflicts and water should be considered as vital to national security as defence, a United Nations report says.
About 145 nations share river basins with their neighbours and need to promote cooperation over a resource likely to be disrupted by more frequent floods and heatwaves, it said.
Water supplies are under increasing stress from a population of more than 7 billion people likely to reach 9 billion by 2050.
The damaging impacts of climate change are most often seen in water, the study said. Floods in Pakistan in 2010 killed almost 2000 people, while droughts in the United States and Russia in recent years have driven up global food prices.
Water-related diseases, from diarrhoea to malaria, kill about 3.5 million people every year, mostly in developing nations. Climate change could worsen the toll in some areas.
Among encouraging signs, the report said Brazil, Paraguay, Uruguay and Argentina had signed a deal in 2010 to cooperate and prevent conflicts over the Guarani Aquifer, which extends over more than 1 million square kilometres.
The World Health Organisation estimates that each person needs between 50 and 100 litres of water a day to meet basic needs, Reuters reports.