Congressional auditors say the country's food supply is vulnerable to terrorist attacks, partly because the government cannot ensure the security of processing plants.

The General Accounting Office issued the report Tuesday as the Agriculture Department told food companies, retailers and farmers to increase security because of the heightened security alert.

The Food and Drug Administration issued additional security guidelines to food companies Wednesday. Among the recommendations: employee background checks, banning workers from bringing purses or other personal items into food-storage areas and training workers to recognize suspicious behavior or possible food tampering.

The FDA also increased inspections of certain food factories and of imported foods.

The congressional auditors said that USDA and the FDA, which share oversight of the food supply, could lower the risk of an attack if food companies had to share detailed security plans with them.

Now, companies are asked to volunteer the information.

"Without the ability to require food processing facilities to provide information on their security measures, these federal agencies cannot fully assess industry's efforts to prevent or reduce the vulnerability of the nation's food supply to deliberate contamination," said the report by GAO, the investigative arm of Congress.

Garry McKee, administrator of food safety at USDA, said the agency generally agreed with the report. FDA officials said their agency has laws in place to protect food from tampering.

Manufacturers disagree with GAO's suggestion that they be required to share security plans with regulators. That would make them susceptible to terrorist attacks, they argue, because their plans could be released to the public.

The voluntary system is better because "it would allow flexibility to change, to fit the evolving needs and also to protect the security of information," said Tim Willard, spokesman for the National Food Processors Association.

Also on Tuesday, food companies and farmers were asked to increase security because the nation's terror level was elevated from yellow to orange. The orange alert indicates a high risk of attacks in the United States or against American interests abroad. The government is especially concerned as the nation prepares for a possible invasion of Iraq.

Agriculture Secretary Ann Veneman said USDA experts "have provided security guidelines for producers, processors and food providers in order to strengthen systems at the local level."

The department also increased security at crucial USDA buildings, such as laboratories. Veneman said emergency response teams at the department are on standby in case of an attack.

Public events and tours at USDA sites have been restricted.

The agency improved food security after Sept. 11, 2001, by hiring 20 additional inspectors to check imported meat for contamination. Veneman also said the agency developed a network of laboratories to help detect and diagnose diseases, such as anthrax, which terrorists could use as a weapon.