U.S. officials placed a hold on 20 million chickens raised for market in several states because their feed was mixed with pet food containing an industrial chemical.

Three government agencies — the Agriculture Department, the Food and Drug Administration and the Environmental Protection Agency — are determining whether the chickens would pose a threat to human health if eaten, USDA spokesman Keith Williams said. The assessment may be completed as early as Monday.

The 20 million chickens represent a tiny fraction of the 9 billion chickens raised each year in the United States. Meat from the birds cannot go into commercial use without the USDA's inspection seal, which is being withheld until the risk assessment is completed, Williams said.

The states with chicken producers affected by the hold will be announced later, Williams said.

Since March 16, more than 100 brands of pet food have been recalled because they were contaminated with melamine. An unknown number of dogs and cats have been sickened or died after eating pet food tainted with the chemical, which contaminated wheat gluten in the food imported from China.

Investigators found last week that about 5 percent of feed used at some smaller chicken production operations came from pet food tainted with melamine, Williams said. Larger manufacturers are unlikely to have exposed their animals to large amounts of the tainted pet products because they usually use special feed, he said.

As of Friday, no melamine had been detected in the feed used by larger manufacturers, Williams said. However, because investigators know some of the tainted pet food was used in that feed, officials still placed a hold on the birds, he said.

The Agriculture Department's Food Safety and Inspection Service said Thursday that no evidence indicated any harm to humans from chicken or pork that had entered the market after having eaten melamine-contaminated feed.

Federal investigators have been trying to determine how much of the tainted pet food had been used in feed for hogs and chickens. Hog farms in at least six states may have received tainted pet food for use in feed. Those animals also have been barred from market.