The approval process has been on hold since 1998 when six EU governments, led by France, said they would not allow any new GMOs into the 15-nation bloc until tougher rules on testing, labelling and tracing were adopted.

In its opinion, the AFSSA agency said current rules on GMOs did not offer complete health guarantees for consumers and the agency raised questions about the possible side-effects of long-term exposure to such products.

"It is essential that precautions be taken to limit as much as possible any risks of allergies to transgenic products," AFSSA said in a statement.

The agency noted that while GMOs were tested to see if they caused immediate allergic reactions, it was necessary to test laboratory animals to determine whether there were any long-term effects from eating gene-spliced crops.

"Only such tests will show potential effects of prolonged exposure on vital systems - notably the immune, hormonal and reproductive systems," the agency said.

AFSSA also said it would back tests on humans of any GMOs designed to improve health, but that such tests regarding gene-spliced crops designed simply to boost farm yields would pose "ethical problems".

EU Trade Commissioner Pascal Lamy acknowledged last week in Washington that the EU's moratorium on approvals was a source of frustration for the United States and other EU trading partners.

Companies like Monsanto Co and Novartis AG have been waiting for years to know whether their new strains of modified maize, soy and cotton can be sold in the EU.