Billions of dollars pledged by developed nations in climate finance over the last decade remain unused as poor countries that often most need the money are ill-equipped to spend it, international charity WaterAid said on Tuesday.

Despite donors' willingness to provide funding, vulnerable countries worst hit by rising climate pressures such as droughts and floods often lack the capacity to implement, manage and oversee projects to help them cope, WaterAid said.

"Countries that are most in need of adaptation finance are also those that are least able to spend it," WaterAid analyst Louise Whiting told the Thomson Reuters Foundation.

More support is required to help countries get ready to use climate finance "so they can actually spend the money and get it to where it is needed", she added.

Just 15 percent of the $21.1 billion in climate finance approved by developed countries from 2003 to 2014 has been disbursed, and little of it went to the world's poorest countries, WaterAid said in a report based on research by development consultancy Oxford Policy Management.

Sub-Saharan African countries received less than 2.5 percent of total climate funding in that period despite being among those at greatest risk from extreme heat and erratic rainfall, and being responsible for only a tiny fraction of planet-warming emissions, the report said.

Just 16 percent of climate finance has been used for adaptation programmes, while the vast majority of the money is being spent on reducing carbon emissions in middle-income countries, it said.

WaterAid joined other anti-poverty charities in urging world leaders due to agree a new deal to curb global warming at a U.N. summit in Paris next month to boost funding for efforts to adapt to climate change impacts.

WaterAid said it was alarming that, despite growing evidence linking climate change to water security, only a fraction of climate finance goes to securing access to clean water during floods or droughts, jeopardising development gains.

"For families living in extreme poverty, with fragile access to safe water, good sanitation and hygiene, these lengthening dry seasons and intensifying monsoons wipe out years of work and further entrench the cycle of poverty," Whiting said in a statement.

"Safeguarding basic services including clean water, sanitation and hygiene helps communities recover faster and become more resilient to climatic extremes."

More than 650 million people around the world live without access to clean water and 1.2 billion live in areas of water scarcity, WaterAid said, citing United Nations' figures.

They will become even more vulnerable as their already scarce resources are further exposed to climate-related threats, WaterAid said.