China has suspended imports from several major U.S. meat processors, including the world's largest, in the latest indication the government may be retaliating as its products are turned back from overseas because of safety concerns.

Frozen poultry products from Tyson Foods Inc. (TSN), the world's largest meat processor, were found to be contaminated with salmonella, the General Administration of Quality Supervision, Inspection and Quarantine said on its Web site late Friday.

Other imports barred by China included frozen chicken feet from Sanderson Farms, Inc. (SAFM) tainted with residue of an anti-parasite drug, as well as frozen pork ribs from Cargill Meat Solutions Corp. containing a leanness-enhancing feed additive, the AQSIQ said.

A spokesman for Cargill denied the agency's claims, while officials at Tyson and Sanderson Farms were not immediately available to comment.

China's food and drug safety record has come under scrutiny in recent months following the deaths of cats and dogs in the United States and Canada blamed on tainted Chinese pet food ingredients. Worries at home and around the world have heightened as a growing number of Chinese products are found tainted with dangerous levels of toxins and chemicals.

Beijing has taken significant steps in recent days to clean up its product safety record, including executing the former head of its drug regulation agency for taking bribes and banning the use of a chemical found in antifreeze in the production of toothpaste.

Officials also have vowed to better integrate its fractured regulatory system, which splits responsibility among at least six agencies. The blurred lines often enable the country's countless illegal operations to escape detection.

The Chinese government has thoroughly investigated each case of substandard products, said Li Yuanping, director of the AQSIQ's import and export bureau, the official Xinhua News Agency reported Saturday.

"All of them are exceptional cases," he said, noting that more than 99 percent of China's exports meet standards.

"There is no such thing as zero risk. In term of food safety, it's impossible for any country to make 100 percent of their foodstuff safe," he said. "China-made products should not be labeled as substandard just because of a few bad producers."

But as China works to improve its food safety conditions, authorities also have prominently announced rejections of imports, apparently to turn the tables on critics and show it is not the only country with food export safety problems.

Mark Klein, a spokesman for Minneapolis-based Cargill Inc., disputed the Chinese inspectors' findings and said the company hoped to resolve the issue by working with U.S. and Chinese officials.

"We're proud of our products and our processes, and we'll be delighted to talk about them with all concerned," he said.

Cargill is the parent company of Wichita, Kan.-based Cargill Meat Solutions Corp., which as of 2005 was the ninth leading pork producer in the U.S., according to the National Pork Producers Council.

Cargill had 45 days to correct the problem, the AQSIQ said, as did Kansas City, Mo.-based Van Luin Foods USA, Inc., whose imports of frozen pig ears also were found to contain the leanness-enhancing feed additive ractopamine.

The other companies' imports were suspended, although the AQSIQ did not say for how long. It also was not clear whether the latest import bans covered only the products in question or if they extended to all of the companies' imports. A duty officer at the AQSIQ on Saturday said he did not know any details.

Also suspended from import were products from two Atlanta-based companies: frozen chicken feet tainted with salmonella from Intervision Foods and frozen pork tainted with ractopamine from AJC International, Inc. Phones at both companies rang unanswered late Friday.

Messages seeking comment were left with two spokemen for Springdale, Ark.-based Tyson. A voicemail seeking comment was left at Laurel, Miss.-based Sanderson Farms Inc., which is among the top five poultry producers in the U.S., according to the company's Web site.

Van Luin Foods could not be reached for comment.

Beijing has previously rejected shipments of substandard orange pulp, dried apricots, raisins and health supplements from the U.S.