US food watchdog the Food and Drug Administration (FDA) has blocked imports of certain farmed seafood products from China after unsafe residues were found.

The FDA announced a broader import control of all farm-raised catfish, basa, shrimp, dace and eel from China. It said it would be detaining these products at the border until the shipments are proven to be free of residues from drugs that are not approved in the US for use in farm-raised aquatic animals. However, the FDA added that there had been no reports of illnesses to date.

"We're taking this strong step because of current and continuing evidence that certain Chinese aquaculture products imported into the United States contain illegal substances that are not permitted in seafood sold in the United States," said David Acheson, the FDA's assistant commissioner for food protection. "We will accept entries of these products from Chinese firms that demonstrate compliance with our requirements and safety standards."

The FDA said that during targeted sampling from October 2006 to May 2007, it had repeatedly found farm-raised seafood imported from China to be contaminated with antimicrobial agents, namely nitrofuran, malachite green, gentian violet, and fluoroquinolone, that are not approved for this use in the US.

Nitrofuran, malachite green, and gentian violet have been shown to be carcinogenic in laboratory testing. The use of fluoroquinolones in food animals may increase antibiotic resistance to this critically important class of antibiotics, the FDA said.

However, the levels of the drug residues detected were very low, most often at or near the minimum level of detection, and the FDA said it is not seeking to recall products already in the US or advising consumers to destroy any such products they may already have bought.