Castro, who has taken to writing articles since he was sidelined from power last year by intestinal surgery, has attacked US plans to increase biofuels output using crops such as corn, saying this will increase food prices and global hunger.

"What President Castro points to is something the UN Food and Agriculture Organization has also raised recently: That there is significant potential and risk for competition between food production and production for a global biofuels market," Steiner told Reuters during a environmental meeting in Havana.

"We have to be aware that there are risks, and for some countries those risks may not be worth taking," he said.

Steiner said it is too early to do a cost-benefit analysis on the use of ethanol, which environmentalists say will help slow global warming.

While current technology simply turns crops, such as sugar or corn, into ethanol, new biofuels products on the horizon use enzymes to turn crop residue or agricultural waste into fuel, he said.

The UNEP is studying the efficiency of biofuels while focusing on the development of international standards that would minimize social and environmental risks.