A massive recall of eggs possibly tainted with salmonella bacteria is now at more than half-a-billion and could grow, the top US food safety official said Monday.

"It is the largest egg recall that we've had in recent history," Margaret Hamburg, head of the US Food and Drug Administration (FDA), told NBC television.

"We may see some additional recalls over the next couple days, even weeks, as we better understand the network of distribution of these eggs contaminated," she said.

Hamburg added that US officials were "continuing to investigate aggressively to determine the exact source of the contamination as we move forward with the recall."

There are no known salmonella deaths due to the eggs, but health officials said the outbreak has sickened hundreds of people across the United States.

The recalls affect two chicken egg processors -- Hillandale Farms and Wright County Eggs, both in the midwestern state of Iowa -- and cover at least 14 US states, from California to Arkansas.

The US Centers for Disease Control and Prevention (CDC) has warned that the outbreak could actually be larger than reported, because most people who get sick from salmonella do not seek medical treatment.

The FDA said infected rodents many have spread the salmonella to the eggs.

Responding to complaints that the recalls were ordered just last week even though people began getting sick in May, Hamburg said it takes time for any food safety probe to get up and running.

"You have to start an investigation. First you see the rise in the number of cases of salmonella above the normal background. Then you start to identify the cases and do the investigation of where they might have been exposed," she said.

"This salmonella is the most common kind so it makes it a little bit harder to track down the source," Hamburg added.

"We've tried to move swiftly to identify the source and take the action necessary to protect the consumer."

Health experts say salmonella is spread most often by the consumption of food contaminated by animal fecal matter.

The microbe usually flourishes within the intestinal tracts of fowl and mammals.

An estimated 400,000 people are infected with food-borne salmonella each year in the United States, according to the CDC.

Recent years have seen various massive food recalls in the United States -- from salmonella-tainted peanut butter to pistachios to frozen spinach, and milk -- amid criticism that America's food regulation regime is under-staffed and overtaxed.

"We need additional resources. We need additional authority. We need greater ability to trace back products to their source so that we can identify how the contamination occurred and what products are at risk," Hamburg told NBC.

"We need better abilities and authorities to put in place these preventive controls and hold companies accountable, and we need to be able to more routinely review records and work with companies to make sure that the food supply is safe."