WASHINGTON - Food and milk from the offspring of cloned animals may have entered the U.S. food supply, the U.S. government said on Tuesday, but it would be impossible to know because there is no difference between cloned and conventional products.

Gloria, the first calf born to a cloned cow, Vitoria (L), is seen on a government farm outside Brasilia in this October 4, 2004 file photo. (REUTERS/Jamil Bittar/Files) The U.S. Food and Drug Administration said in January meat and milk from cloned cattle, swine and goats and their offspring were as safe as products from traditional animals. Before then, farmers and ranchers had followed a voluntary moratorium on the sale of clones and their offspring.

While the FDA evaluated the safety of food from clones and their offspring, the U.S. Agriculture Department was in charge of managing the transition of these animals into the food supply.

"It is theoretically possible" offspring from clones are in the food supply, said Siobhan DeLancey, an FDA spokeswoman.

Cloning animals involves taking the nuclei of cells from adults and fusing them into egg cells that are implanted into a surrogate mother. There are an estimated 600 cloned animals in the United States.

Proponents, including the Biotechnology Industry Organization, say cloning is a way to create more disease-resistant animals that produce more milk and better meat. The cloning industry and the FDA say cloned animals and their offspring are as safe as their traditional counterparts.

Critics contend not enough is known about the technology to ensure it is safe, and they also say the FDA needs to address concerns over animal cruelty and ethical issues.

"It worries me that this technology is out of control in so many ways," said Charles Margulis, a spokesman with the Center for Environmental Health. The possibility of offspring being in the food supply "is just another element of that," he said.

FDA and USDA have said it is impossible to differentiate between cloned animals, their offspring and conventionally bred animals, making it difficult to know if offspring are in the food supply.

"But they would be a very limited number because of the very few number of clones that are out there and relatively few of those clones are at an age where they would be parenting," said Bruce Knight, USDA's undersecretary for marketing and regulatory programs.

As the FDA unveiled its final rule, USDA in January asked producers to prolong the ban on selling products from cloned animals. That ban did not extend to meat and milk from the clone's offspring.

Major food companies including Tyson Foods Inc, the largest U.S. meat company, and Smithfield Foods Inc have said they would avoid using cloned animals because of safety concerns.

The list grew on Tuesday after the Center for Food Safety and Friends of the Earth said 20 food producers and retailers vowed not to use ingredients from cloned animals.

The list, provided by the two groups, included Kraft Foods Inc, General Mills Inc, Campbell Soup Co, Nestle SA, California Pizza Kitchen Inc and Supervalu Inc.

In a letter to the Center for Food Safety, Susan Davison, director of corporate affairs with Kraft, said product safety was "not the only factor" the company considers.

"We must also carefully consider additional factors such as consumer benefits and acceptance ... and research in the U.S. indicates that consumers are currently not receptive to ingredients from cloned animals," she said.

Editing by Christian Wiessner and David Gregorio