The European Commission ordered on Wednesday that imports of long grain rice from the US be certified free of an unauthorized genetically modified strain.
The EU's executive arm decided to take the action after Washington informed Brussels last week that an unauthorized strain of GM long-grain rice had been detected on the US market.
"We have strict legislation in place in the EU to ensure that any GM product put on the European market has undergone a thorough authorization procedure based on scientific assessment," said EU Health and Consumer Protection Commissioner Markos Kyprianou.
"There is no flexibility for unauthorized GMOs -- these cannot enter the EU food and feed chain under any circumstances," he said. "The measures we have taken today will ensure that unauthorized GM rice is not inadvertently imported."
Last year, the EU imported 198,000 tonnes of long-grain rice from the US, worth 52 million euros (US$67 million), according to the commission.
The US said that it was reviewing EU demands, while insisting that such varieties were perfectly safe.
"This particular rice and the protein in it is not a threat to human health or plant health," USDA spokesman Jim Rogers said.
"USDA is in the process of carefully examining the announcement from the EU. We're going to continue to work with our own rice industry and our trading partners to facilitate good trade," he said.
Under the new requirement, shipments of US long grain rice have to be tested by accredited laboratories, which must provide a certificate showing that the consignment is free of the GM strain.
Although the decision has to be submitted to food safety experts from member states for review, the measures enter into effect immediately and are expected to be reviewed after six months, the commission said.
US authorities and Bayer CropScience, which produced the strain of rice and is the agro-industry arm of the German chemicals and drugs giant, found that the GM rice represented no risk to humans or the environment.
However, environmental groups are outraged that the rice has ended up in the US market and some have called for a full ban on imports of US rice to the EU.
"It is time to move beyond case-by-case procedures as the GE [genetical engineering] industry has shown time and time again that it is unwilling or unable to prevent GE contamination," Greenpeace campaigner Jeremy Tager said.
"A message needs to be sent to the US and to agro-chemical giant Bayer that genetic contamination and `accidents' with our food are not acceptable, and ultimately they must be held liable for cleaning it up," he stressed.
Although attitudes to GM crops and products vary within the EU, they are tightly regulated largely as a result of public unease about the health and environmental impact of them.
In April last year, the European Commission also ordered inspections of some imports of animal feed from the US after discovering an unauthorized GM maize from the US.