U.S. officials are conducting a risk assessment to determine if millions of chickens are safe for human consumption after discovering their feed was contaminated.

The evaluation, drafted by the U.S. Agriculture Department, the Food and Drug Administration and the Environmental Protection Agency, could be completed as early as Monday.

On Friday, the FDA held back 20 million chickens from going to market because their feed contained melamine — a chemical used to make plastics and fertilizers.

Melamine found in tainted pet food has been blamed for the deaths of 16 cats and dogs, according to the FDA, which has also fielded about 17,000 consumer calls about related pet illnesses.

Officials traced the problem to imported rice gluten and wheat protein used in the pet food. More than 100 brands of pet food have been withdrawn from the marketplace since March 16.

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

U.S. hogs also being held

The FDA says that chickens raised at larger plants were not likely given the tainted feed but they said that about five per cent of feed used at smaller production facilities may have been contaminated.

The U.S. agency is also holding back from market hogs from farms in six states that had received tainted pet food for use as feed.

The FDA last week said it would not be issuing a recall on poultry or pork products already sent to market given that the feed comprised only a small portion of the animals' diet and the risk to humans was accordingly low. 

With files from the Associated Press