Pork, chicken and egg products from animals that were given feed tainted with melamine do not pose a health risk to humans, U.S. officials said Monday.

Tests on the feed showed the concentration of melamine — a chemical used to make plastics and fertilizers — was very low and did not pose a significant threat to humans, U.S. government scientists said.

"We literally found that the dilution is so minute, in fact in some cases you can't even test and find melamine any more in that product," Agriculture Secretary Mike Johanns said in Chicago.

Food safety concerns grew after U.S. officials found that some small manufacturing plants had been incorporating pet food into their animal feed before tests showed the pet food was contaminated.

A massive recall involving more than 100 brands was issued on March 16 and the FDA has confirmed the deaths of 16 pets. The FDA has fielded about 17,000 consumer calls about related pet illnesses.

U.S. Food and Drug Administration and Department of Agriculture officials said farm animals were not exposed to the same amounts of melamine as pets. They also noted that pigs and chickens that were given contaminated feed seem to be healthy.

The government on Monday also lifted its hold on 20 million chickens after tests showed their feed was not contaminated.

About 6,000 hogs and a selection of other animals will be held until completion of a risk assessment of their feed, which tested positive for contamination.

Canada's food inspectors have issued border lookouts for vegetable proteins coming from China to prevent melamine from contaminating the human food chain.

With files from the Associated Press