LOS ANGELES — Foreign dog breeders have gone unregulated for years, shipping puppies so young and so sick that 1 in 4 died before reaching a U.S. airport, animal-welfare workers say.

The U.S. Department of Agriculture approved a regulation Friday that, starting in 90 days, will require all puppies imported to the United States to be at least 6 months old, healthy and up-to-date on vaccinations.

Census Bureau data show about 8,400 puppies a year were imported between 2009 and 2013. Because there were no regulations, however, the Humane Society of the United States believes the numbers were much higher, said Melanie Kahn, the society’s director of puppy-mill campaigns.

Many of the puppies came from mills in China and Eastern Europe, said Deborah Press, of the American Society for the Prevention of Cruelty to Animals (ASPCA).

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This is the second major USDA effort regarding puppy mills in the past 12 months. In September, the agency enacted what is called the “retail rule”: Breeders having four or more female breeding dogs have to be licensed if they are selling to consumers sight unseen on websites, in flea markets or in classified ads.

The Humane Society and ASPCA said they routinely get calls from people who unwittingly bought a puppy-mill dog from a foreign or U.S. breeder, only to have it die because of illness.

It has been hard to track the number of puppies that are imported, Kahn said, but, she added:

“We have seen an increase just in the past few months based on calls from consumers who bought teacup puppies from Korea.”

Importers have been sending puppies that are younger than 8 weeks to the U.S. in airliners’ cargo holds, Kahn said.

Under the new regulation, published in the Federal Register on Friday, violators can be fined up to $10,000.

The ban eliminates the easy access to market that foreign breeders have had for years, said Cori Menkin, senior director of the ASPCA puppy-mills campaign.