The UN says hundreds of millions of dollars promised by rich countries to help developing nations deal with climate change cannot be accounted for, Press TV reported.
In 2001, the EU and several industrialized countries pledged more than $400m a year.
However, BBC reported that based on its investigation in eight years, just $260m had been paid into two designated UN funds.
The EU says the money was paid out in bilateral deals, but admits it cannot provide data to prove it.
The money was pledged in the 2001 Bonn Declaration, signed by 20 industrialized nations - the 15 countries that then made up the European Union, plus Canada, Iceland, New Zealand, Norway and Switzerland.
The UN Secretary-General Ban Ki-moon said there was a need to build trust on the issue.
The comments came shortly after scientists issued a sobering review that said the planet could be getting much hotter, much faster than anticipated only two years ago.
The 68-page report suggests that many of the estimates in a report by the UN Intergovernmental Panel on Climate Change in 2007 were too low.
The planet could now become warmer by seven degrees Celsius and sea levels could rise by 3.25 feet by 2100.
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