China has shut down two companies for exporting tainted pet food ingredients believed responsible for poisoning hundreds of dogs and cats in North America. China has been defending itself against accusations of poor quality controls after a series of revelations that some exports were unsafe. As Daniel Schearf reports from Beijing, Chinese officials say lax U.S. customs inspections are also to blame.

A senior quality inspection official Friday said two Chinese companies had their business licenses revoked after an investigation revealed they added the toxic chemical melamine to wheat and rice protein used to make pet food in North America.

Melamine is high in nitrogen and can make products it is added to appear higher in protein than they actually are. But it is also poisonous and was likely the cause of hundreds of pet deaths earlier this year in the United States and Canada.

Li Changjiang, the director of China's General Administration of Quality, Supervision, Inspection and Quarantine says the companies were able to evade quality controls by mislabeling the products.

"According to our laws, chemical ingredients are not subject to inspection," Li explained. "So, they were not inspected. These companies were responsible because they were not supposed to pass off a chemical ingredient as a product for export. Therefore, we severely punished these enterprises."

The quality inspection agency said the powerful Public Security Bureau has started an investigation and arrested "major personnel" in charge.

The pet deaths were followed by discoveries in several countries of tainted Chinese toothpaste, fish, and children's toys, sparking international concern about Chinese exports.

The U.S. has refused farmed fish and seafood imports from China after shipments were found to be contaminated with drugs banned in the U.S.

Li said those tainted shipments came from illegal fish farms. He said U.S. customs were also at fault because, unlike the European Union and Japan, the U.S. does not require exporters to provide official certificates of health.

"They allow all products to be exported and then conduct random tests in ports," Li said. "This makes it easy for some illegal enterprises' food products that do not meet U.S. standards to enter the U.S. market."

Li defended Chinese food products. He said large and medium sized food manufacturers with high standards made up 90 percent of the market. He vowed to crack down on China's numerous small food manufacturers scattered around the country, which he blamed for most food safety problems.

Chinese officials have chastised foreign media for giving Chinese products a bad name. Li said U.S. media reports "untruthfully" linked poor quality Chinese-made tires to a fatal car accident, leading to a recall of nearly half a million tires. Chinese officials have concluded misuse of the tires was the real cause of the accident.