Approximately one in four children worldwide will live in regions with extremely scarce water resources by 2040, UNICEF said in a report Wednesday.
In research released on World Water Day, the United Nations children's agency warned that in just over two decades nearly 600 million children will be living in areas with severely limited safe water sources, as population growth and surging demand for water clash with the effects of climate change.
More than 36 countries are currently enduring extreme water stress, the report said, with water demand exceeding available renewable supplies.
Rising temperatures and droughts can leave children at risk for dehydration, according to UNICEF, while increased rain and flooding can destroy sanitation infrastructure and help spread water-borne diseases like cholera.
More than 800 children under the age of five die every day from diarrhea linked to poor sanitation and scarce clean water sources, the report said.
Drought and conflict are factors behind water scarcity in parts of Nigeria, Somalia, South Sudan and Yemen, the report says. Nearly 1.4 million children in those areas facing "imminent risk of death" from famine.
More than 9 million people in Ethiopia alone will lack access to safe drinking water this year, according to agency projections.
Without water "nothing can grow," said UNICEF Executive Director Anthony Lake.
"Millions of children lack access to safe water -- endangering their lives, undermining their health, and jeopardizing their futures."
The crisis "will only grow unless we take collective action now," he said.
UNICEF urged communities to diversify water sources, and for governments to prioritize access to safe water for vulnerable children.
In another report released Wednesday, the UN said recycling the world's wastewater -- almost all of which goes untreated -- would ease global water shortages while protecting the environment.
Two-thirds of all humans live in areas that experience water scarcity at least one month a year -- half of them in China and India.
Last year, the World Economic Forum's annual survey of opinion leaders identified water crises as the top global risk over the next decade.
On current trends, the UN Environment Program forecasts that water demand -- for industry, energy and an extra billion people -- will increase 50 percent by 2030.