The US is holding back a rare cargo of Polish wheat being imported into Houston, a move that may spark tensions with the European Union, according to people familiar with the matter.

The vessel Yochow, carrying about 30,000 tons of Polish wheat, is being prevented from unloading at the port of Houston, according to shipping data and the people, who asked not to be identified because the information is private. Authorities are citing contamination with corn, the people said.

The US is resorting to purchases of European wheat after a drought pushed local prices higher. While there are enough American supplies for the nation’s flour mills, it’s cheaper to bring in grain from Poland than to haul it from the Midwest to places like Texas and Florida.

The rare imports are a blow to US farmers, which have been losing power in the global market for years to top shipper Russia. A final rejection of cargo could spark tensions with the EU. The US has in the past accused other countries of imposing non-tariff barriers to trade when American cargoes were rejected in places like China.

US Customs and Border Protection didn’t provide a comment. The Polish Grain and Feed Chamber didn’t have an immediate comment when contacted by phone, and a European Commission spokesperson declined to comment.

The Yochow left Poland on July 13 and arrived in Houston on Aug. 6, according to ship-tracking data. EU data shows two cargoes — one carrying about 29,000 tons and another 33,000 tons — sailed from Poland to the US in July.