Growing demand for energy will put increasing pressure on the world's already strained water resources, particularly in developing and emerging economies, the UN has warned.

"There is an increasing potential for serious conflict between power generation, other water users and environmental considerations," it says in the world water development report, published on the eve of world water day on Saturday.

Energy production accounts for close to 15% of the world's water usage, but that figure could rise. By 2035, water use for energy is projected to jump by 20%, the report says. Water demand, meanwhile, could increase by 55% by 2050.

Much of this is due to growing populations and economies in China, India and the Middle East, says the report, which pulls together data from a range of studies. Some 90% of the global increase in demand for energy in the coming years will come from outside the Organisation for Economic Co-operation and Development (OECD), club of rich countries, it notes.

"Demand for fresh water and energy will continue to increase over the coming decades to meet the needs of growing populations and economies, changing lifestyles and evolving consumption patterns, greatly amplifying existing pressures on limited natural resources and on ecosystems," the report says.

About 90% of power generation is water-intensive, says the report, which warns that less conventional oil and gas production, including via tar sands and fracking – along with biofuels – place particularly large demands on water resources.

The report laments that the energy sector has "great political clout", whereas water, as an issue and an industry, often lacks influence; it calls for greater co-ordination between the two areas. "There will be no sustainable development without better access to water and energy for all," the director general of Unesco, Irina Bokova, said.

Estimates suggest 768 million people are without clean drinking water, and 2.5 billion lack access to even basic sanitation. Some 1.3 billion are not connected to power grids, and close to 2.6 billion people use solid fuel, mainly biomass, to cook.

In a separate report (pdf) , also published on Friday, the NGO WaterAid accuses rich aid donors of failing to deliver billions of dollars in promised aid for water and sanitation over the past decade.

Data compiled by the OECD and analysed by WaterAid suggests donors promised more than $80bn (£48bn) in aid for water and sanitation in developing countries between 2002 and 2012. However, they delivered only $53.6bn – a shortfall of $27.6bn.

Water and sanitation received just 6% of total donor aid in 2012, says WaterAid. Photograph: AP

The WaterAid chief executive, Barbara Frost, said: "If the world's promises on aid were met … [this] could have helped transform the lives of millions of people around the world. But just meeting existing commitments is not enough," she said. "Overall aid flows for water and sanitation need to double to create the step change in access to these essential services."

According to WaterAid, water and sanitation received just 6% of total donor aid in 2012. The NGO also questioned how donors were allocating their aid for water and sanitation, saying that money was failing to reach the neediest and might be exacerbating global inequalities instead.

WaterAid's calculations suggest that Mauritius, for example, where more than 90% of the population has access to water and sanitation services, receives $588 in aid per person, while in the Democratic Republic of the Congo, where more than half the population goes without clean drinking water and basic sanitation, that figure is just $0.80.

"The stated aim of international aid is to help the world's poor break out of poverty and to live healthy and productive lives – and to positively address our fundamentally unequal world," Frost said. "The question has to be asked: why is the majority of water and sanitation aid not being targeted at those who are desperately waiting for these essential services?"

The target contained in the millennium development goals to halve the proportion of people without access to improved water sources was met years before the 2015 deadline. But still, "the overall picture is one of a major divide: abundance, even excess for some, yet scarcity, or even complete absence of safe water for others. Those on the wrong side of the divide face a daily struggle," the report says.