UN Secretary-General Ban Ki-moon has expressed concern at a report that hundreds of millions of dollars promised by rich countries to help developing nations cope with climate change cannot be accounted for.
In 2001, the EU and several industrialised countries pledged more than $400m a year.
However, a BBC investigation has found that in eight years, just $260m had been paid into two designated UN funds.
Mr Ban said there was a need to build trust on the issue.
Separately, US President Barack Obama has expressed hope that agreement can be reached at December's Copenhagen climate summit, as a new report showed the crisis facing the planet is deeper than previously thought.
Mr Obama said recent progress meant the world was 'one step closer to a successful outcome in Copenhagen.'
Countries must 'reach a strong operational agreement that will confront the threat of climate change while serving as a stepping-stone to a legally binding treaty,' he told a press conference with Indian Prime Minister Manmohan Singh.
In a joint statement, Mr Singh and Mr Obama spoke of their hopes for a 'comprehensive' deal in Copenhagen.
'It should reflect emission reduction targets of developed countries and nationally appropriate mitigation actions of developing countries,' they said.
The upbeat 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 warm by seven degrees Celsius and sea levels could rise by 3.25 feet by 2100.
Germany's Potsdam Institute for Climate Impact Research oversaw the report.
