The official, Franz Perrez, was speaking at a news conference on the eve of a two-day gathering of environmental ministers and experts from some 45 countries to discuss how to reach agreement on a funding deal.

"An agreement on viable long-term financing is one of the very important building blocks for a new convention to combat the challenge of climate change," said Perrez, whose country has organised the informal meeting together with Mexico.

In December, Mexico is to host a new formal effort to clear the way for a convention. A United Nations summit in Copenhagen at the end of last year ended in serious disarray.

Developing nations say billions of dollars are vital to help them start acting to slow global warming by shifting from fossil fuels, and to cope with challenges created by climate change ranging from droughts and floods to rising sea levels.

Big emerging economies like China, India and Brazil say they should not be hog-tied by environmental rules unless the West -- which they blame for global warming -- helps pay the cost.