Two American experts have refuelled the controversy over genetically modified (GM) crops by saying no one can be certain of the risks or benefits.

The scientists, from the US Environmental Protection Agency, said that genetic engineering and selective breeding may not have the same long-term effects.

"As more economically useful and health-related genes are identified and isolated, it appears that the variety of genetically engineered organisms will increase dramatically. This increase may collectively represent an environmental risk," wrote LaReesa Wolfenbarger and P. R. Phifer, in the journal Science.

The US Environmental Protection Agency is currently involved in a dispute with the US Department of Agriculture, which has allowed the commercial planting of thousands of acres of GM crops such as soya and maize.