* Shared rivers, aquifers face strains from more demand
* 145 countries share river basins with neighbours
* U.N. issues first definition of water security
By Environment Correspondent Alister Doyle
OSLO, March 22 (Reuters) - 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 said on Friday.
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.
"In the past few decades, definitions of security have moved beyond a limited focus on military risks and conflicts," Michel Jarraud, chair of U.N. work on water and head of the World Meteorological Organization, said in the report.
About 185,000 Somalis fled to neighbouring nations in 2011, driven largely by water and food shortages linked to drought, while in South Sudan, entire communities were forced to leave due to water scarcity brought on by conflict in 2012.
"Few issues ... have the potential to create friction more than the management of water shared across international borders," said former Canadian Prime Minister Jean Chretien, who chairs a group of 37 former heads of government campaigning to make water a security issue.
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 2,000 people and 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.
The report said that watersheds - lines that separate neighbouring drainage basins - cross the territories of 145 nations, and there are over 300 trans-boundary aquifers from which groundwater can be extracted.
"Trans-boundary waters pose enormous challenges for achieving water security," the report said.
Among encouraging signs, it 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 (386,000 sq miles).
The United Nations issued a first working definition of water security on Friday - sustainable supplies to ensure human wellbeing, avert water-related disasters, conserve ecosystems and aid economic and social development.
"This definition is a starting point," said Zafar Adeel, co-chair of the U.N. water task force on water security.
The World Health Organization estimates that each person needs between 50 and 100 litres (13-26 U.S. gallons) of water a day to meet basic needs. (Reporting By Alister Doyle; Editing by Tom Pfeiffer)