Concerns about genetically modified organisms prompted Mexican officials to stop shipments of U.S. rice at the border, although they were eventually released and allowed to enter after intervention by the U.S. Department of Agriculture, USDA officials said.

USDA spokesman Keith Williams said at least two shipments of U.S. rice were held up until USDA Undersecretary Mark Keenum spoke to federal officials there. Mexican officials were demanding the shipments be certified "GMO-free," said a USDA official who asked not to be named in this story.

Mexico agreed to allow the U.S. rice shipments Wednesday, the USDA official said, but Mexican officials are still pushing for the U.S. to come up with some form of non-biotech certification for future exports.