A company at the centre of a salmonella outbreak that has sickened more than 500 people was broadening its recall of peanut products, US health officials said.

The Food and Drug Administration said Peanut Corp of America would expand its recall to all peanut products produced at its Blakely, Georgia plant since January 1, 2007, after agency inspectors found more strains of salmonella at the plant.

"These additional products are being recalled because there is concern of potential salmonella contamination, including contamination with salmonella strains not associated with the current outbreak," Stephen Sundlof, director of the FDA's Center for Food Safety and Applied Nutrition, told reporters in a telephone briefing.

The company said in a statement it was working with the FDA to identify all potentially harmful products and to find ways to improve its manufacturing processes.

Sundlof stressed that to date the only illnesses linked to salmonella in peanut products were caused by the Salmonella Typhimurium strain.

"CDC (Centers for Disease Control) and FDA will continue to monitor incidents of salmonella illness throughout the country," he said.

The expanded recall now includes all peanuts -- dry and roasted -- granulated peanuts, peanut meal, peanut butter and peanut paste made at the Blakely facility, which has stopped production of all products, Sundlof said.

Michael Rogers, head of FDA's field investigations, said the company's internal records revealed situations in 2007 and 2008 where the firm's own testing program had identified salmonella contamination in a product. But, after getting a subsequent negative test, the company resumed shipping products, Rogers said.

"The additional salmonella strains discovered at this plant underscore that this plant was shipping adulterated product, but as for now we are not aware of any illnesses connected to any other salmonella strains shown at this facility," Rogers told reporters.

The FDA's plant inspection report revealed a number of deficiencies, including evidence of cracks in the floor, live cockroaches, mold and water dripping from the ceiling in an area where finished products were stored.

Inspectors noted that the plant lacked adequate facilities for hand washing and a sink in the peanut butter room was used interchangeably for cleaning hands, utensils and washing mops.

The company said it does not agree with all of the observations in the report and said it uses two "highly reputable labs" for product testing.

"PCA categorically denies any allegations that the company sought favorable results from any lab in order to ship its products," it said in a statement.

Dr Robert Tauxe of the US Centers for Disease Control and Prevention confirmed that none of the cases in the current outbreak involve any strain other than Salmonella Typhimurium strain, and said CDC "is not seeing an increase in cases caused by these other types of salmonella."    

Tauxe said the most recent illness occurred on January 9. "The outbreak is still ongoing although the number of reported cases has decreased modestly," Tauxe said.

So far, 501 people from 43 states and one person in Canada have been sickened in the outbreak, 108 people have been hospitalized and there have been eight reported deaths.

FDA officials said it is not yet clear how many more products will be included in the expanded recall. "There is certainly a potential that companies will have to go back further in time to recall products," Sundlof said.

He said the FDA was working with the company and industry groups to identify what products might be involved. -Reuters