The European Union has voted to block certain US grain imports unless they are guaranteed free of a controversial genetically modified strain of corn.
Greenpeace has called for a temporary blanket ban
Some scientists fear the Bt10 strain, which has been entering the EU in a mix with other varieties, could render the antibiotic ampicillin useless.
EU Health Commissioner Markos Kyprianou said the block was necessary to uphold consumer confidence.
Washington's mission to the EU called it an over-reaction.
"US regulatory authorities have determined there are no hazards to health, safety or the environment related to Bt10," a spokesman said.
Bt10 corn apparently entered the EU after being mistaken for Bt11, another genetically modified organism (GMO) which has been authorised by Brussels.
The manufacturers of the corn, Syngenta of Switzerland, said they were doing everything possible to ensure that the safety of humans, animals and the environment was not affected.
From now on, US consignments of corn gluten animal feed and brewers' grain will have to be accompanied by an analytical report from an accredited laboratory.
But Syngenta have not so far come up with a test to distinguish between the banned strain and approved strains.
The environmental organisation Greenpeace said that that could necessitate a blanket ban on the trade in US corn gluten feed, which was worth $450m last year.
Greenpeace GMO expert Christoph Then said Europe was "currently helpless to defend itself" from contamination by GMOs.
"As long as EU authorities have no means to test imports for all the GMOs being released in the US and elsewhere, it must say 'No Entry to the EU' for any food, feed or seeds that are at risk of contamination with illegal GMOs," the Greenpeace expert said.