(Recasts, adds details, background)

By Lisa Richwine and Christopher Doering

WASHINGTON, May 30 (Reuters) - Two companies are recalling livestock and fish feed ingredients because they contain the same chemical linked to the deaths of cats and dogs from tainted pet food, U.S. health officials said on Wednesday.

The presence of melamine and related compounds in feed ingredients sold by Uniscope Inc. and a unit of Tembec Inc. <TBC.TO> was unrelated to the pet food contamination by ingredients from China, the Food and Drug Administration said.

Officials said Tembec confirmed it intentionally added melamine to improve the binding properties of ingredients used to make feed pellets. Melamine is not an approved feed additive.

Investigators are unsure how many animals may have consumed tainted feed. Officials said melamine levels were low and it was unlikely humans would be harmed.

"The companies have stopped adding melamine to the feed product," Dr. David Acheson, FDA assistant commissioner for food protection, told reporters. "FDA is advising feed manufacturers and others who mix their own feed not to use these products, and to contact the manufacturers."

Acheson said Ohio-based Tembec BTLSR, a unit of the Canadian wood products company Tembec, recalled AquaBond and Aqua-Tec II feed ingredients, which it distributes for Uniscope. Colorado-based Uniscope also recalled its Xtra-Bond product that was made with ingredients supplied by Tembec.

The products are binding agents used to make feed pellets for cattle, sheep, goats, fish and shrimp.

Tembec said it was cooperating with the FDA.

"We discontinued the use of melamine in our formulation earlier this spring," Tembec executive vice president John Valley said, adding "we're talking about a low concentration of an element that is in turn a low component of any feed product."

Uniscope originally notified the FDA about the contamination on May 18, agency officials said.

"We don't think the concentration is very high," said Charlie Russell, a spokesman for Uniscope. "But if FDA thinks it's a big deal I'm not sure we're going to disagree with it."

The FDA advised feed manufacturers to recall finished feed made with AquaBond or Aqua-Tec II. But it said no recall was warranted for finished feed made with Xtra-Bond because the levels of melamine and related compounds were lower.

Melamine is an industrial chemical used to make plastics and fertilizer. It had previously been discovered in vegetable protein concentrates from China that were used to make U.S. feed for pets, pigs, chickens and fish.

A federal government probe began in mid-March following reports of kidney failure in some dogs and cats. Pet food makers that used ingredients from two Chinese companies recalled more than 100 brands of melamine-laced feed. Sixteen dog and cat deaths were confirmed among thousands of reports.

A portion of the pet food was used in feed given to poultry, fish and hogs. While some of the animals may have entered the food supply, U.S. officials said it was unlikely that consuming them would pose a risk to human health.

Asked how long Tembec may have been adding melamine to its products, Acheson said "current thinking is this was not a practice that was done very recently but has probably been going on for a little while. We suspect this is not something that happened acutely in May 2007."

David Elder, director of the FDA's enforcement office, said the agency was investigating Tembec's actions and would consider if any action should be taken.

Acheson said some of the feed recalled on Wednesday may have been exported, and U.S. officials were notifying affected countries.

AquaBond and Aqua-Tec II ingredients were used in fish and shrimp feed both in the United States and overseas, while Xtra-Bond was used only domestically in livestock feed. (Additional reporting by Julie Vorman in Washington)

AlertNet news is provided by