Dec 14 (Reuters) - Chewy and eight other companies are violating federal law by selling or making unapproved antibiotics and other antimicrobial drugs for animals that could potentially give rise to drug-resistant superbugs, the U.S. health regulator said.

The U.S. Food and Drug Administration on Thursday sent letters to nine companies including e-commerce retailer Chewy warning them against selling the products that it said contained antibiotics such as penicillin and amoxicillin.

The products, which included brands like Aqua-Mox, Aqua-Zithro, Doxycycline and Fishbiotic, were being sold for so-called minor species like birds and fish.

"Inappropriate use of medically important antimicrobials contributes to the development of antimicrobial resistance, which affects both human and animal health," Tracey Forfa, director of the FDA's veterinary medicine center, said in a statement on Thursday.

Global health officials have repeatedly warned about the rise of antimicrobial-resistant microbes due to the misuse and overuse of antibiotics, which encourages microorganisms to evolve into "superbugs."

Chewy, and four other companies listed in the letter did not immediately respond to Reuters' requests for comment. Reuters could not immediately contact Silver Lease, Valley Veterinary Clinic, American Aquarium Products and Midland Veterinary Services.

The drugs being sold are not approved, neither are they listed on the FDA's index for unapproved animal drugs for minor species, the main legal pathways for animal drugs to reach the marketplace.

The FDA also said that using unapproved animal drugs in humans could delay effective treatment and allow infections to become severe and resistant to antibiotics and anti-fungal drugs.

Anyone using the products should safely dispose them, the agency said. (Reporting by Pratik Jain in Bengaluru; Editing by Maju Samuel)