WASHINGTON - U.S. officials warned Thursday that some contaminated pet food may still be on the shelves in the United States, as a senator complained that a Canadian firm waited too long to notify U.S. authorities about problems with its products.

Senator Dick Durbin of Illinois told a Senate subcommittee that Menu Foods, based in a Toronto-area suburb, waited almost three and a half weeks to inform the U.S. Food and Drug Administration after first noticing test animals were getting sick and refusing to eat their food.

"Why did it take so long?'' asked Durbin, who requested the hearing.

"I think that companies that unnecessarily delay reporting and endanger human and animal health should face penalties, severe penalties.''

Durbin, who advocates one single food safety agency, complained there are too few pet food industry inspections and the system is an unreliable patchwork.

"There are significant health implications to this broken system _ illness, death, lost economic activity and health care costs,'' said Durbin.

FDA officials said they still can't confirm all of the tainted pet food has been pulled. They asked retailers to be vigilant in removing all products associated with the recall.

"We do believe we've got the vast, vast majority off the market,'' said Stephen Sundlof, director of the agency's Center for Veterinary Medicine.

"We know that there is not 100 per cent of the product off the shelf.''

He told the panel that less than one-third of pet food processors in the United States have been inspected in the last three years.

Menu Foods was asked to attend the hearing but it requested that Pet Food Institute, an industry trade association, appear instead.

The institute's president, Duane Ekedahl, told senators that pet food is already perhaps the most highly regulated product on store shelves.

"Pet foods are safe,'' he said, adding that the industry is forming a commission with government officials to investigate how the pet food became tainted and recommend ways to improve safety.

"The answer to this problem is not additional regulation, rather it is enhanced communication,'' he said in written testimony.

On March 16, Menu Foods recalled 60 million cans of dog and cat food produced at their Kansas City and New Jersey plants after the deaths of 16 pets, mostly cats, that ate its products.

The FDA said tests indicated the food was contaminated with an industrial chemical, melamine, used to make plastics and pesticides.

At least six pet food companies have now recalled products made with imported Chinese wheat gluten tainted with the chemical. The recall involved about one per cent of the U.S. pet food supply.

Menu Foods extended its list of recalled products Tuesday, for the first time including foods manufactured at its site in Streetsville, Ont. The recall now includes some 100 brands.

PetConnection.com, a web site tracking the recall, said Thursday more than 3,900 pets have died. A U.S. pet hospital network suggests up to 39,000 animals have become sick.

Elizabeth Hodgkins, a veterinarian, said pet food labels should not be able to make safety claims without rigorous ingredient testing by the manufacturer or the company that supplies the manufacturer.

She said that kind of testing doesn't occur.

"The pet food safety crisis is not an unfortunate aberration but part of mounting evidence of a systemic breakdown,'' Hodgkins said.