The 15-nation EU ended its five-year moratorium on genetically modified food last month but said each product must undergo a scientific risk assessment and have a special label.
U.S. producers and sellers of genetically modified food assert that the labels, which they fear will scare consumers, violate international trade accords and aren't necessary.
The United States, Canada and Argentina formally requested a dispute panel at the Geneva-based World Trade Organization to force the European Union to approve genetically modified foodstuffs unconditionally.
The European Union blocked the request for a special panel, but the United States is expected to raise the issue again on Aug. 29 and the EU won't be able to block it again under trade rules.
The European Union defends its rules as a legitimate response to consumer concerns.
"The EU's regulatory system for genetically modified organisms is clear, transparent, reasonable and nondiscriminatory," EU Trade Commissioner Pascal Lamy said.
If the United States prevails at the World Trade Organization, the union may have to allow imports of the genetically modified crops or compensate trading partners for banning them.
The United States and others might be allowed to impose trade sanctions equal to the amount of sales lost during the European moratorium. American farmers estimate the EU restrictions have cost them nearly $300 million a year in lost corn exports alone.