WASHINGTON — More than two years after Congress passed a landmark law meant to prevent the importation of contaminated food that sickens Americans, the Food and Drug Administration proposed rules on Friday that for the first time put the main onus on companies to police the food they import.

Major food importers and consumer advocates generally praised the new rules, but the advocates also said they worried the rules might give the companies too much discretion about whether to conduct on-site inspections of the places where the food is grown and processed. They said such inspections must be mandated.

The law itself was grappling, in part, with problems that have grown out of an increasingly globalized food supply. About 15 percent of food that Americans eat comes from abroad, more than double the amount just 10 years ago, including nearly two-thirds of fresh fruits and vegetables. And the safety of the food supply — foreign and domestic — is a critical public health issue. One in every six Americans becomes ill from eating contaminated food each year, Dr. Margaret A. Hamburg, F.D.A. commissioner, estimated. About 130,000 are hospitalized and 3,000 die.

The F.D.A. has tried to keep tabs on imports, but, in reality, manages to inspect only 1 to 2 percent of all imports at American ports and borders.