A salmonella outbreak in raw chicken has sickened nearly 300 people in the United States, officials said Wednesday as consumer advocates warned the federal shutdown was hampering a government response.

The 278 illnesses in 18 states were believed to be linked to raw chicken from three Foster Farms locations in California, according to the Food Safety and Inspection Service.

"The outbreak is still ongoing, and therefore our investigation is still ongoing," an FSIS spokesman told AFP, noting that 135 inspectors were on duty responding to the outbreak despite the government shutdown.

"The only folks that were furloughed were back at our headquarters."

The shutdown, which began October 1, has sent hundreds of thousands of federal workers home without pay, including staff at the Centers for Disease Control and Prevention, Department of Agriculture and Food and Drug Administration.

It is the first shutdown in 17 years and was driven by a resurgence of Republican opposition to President Barack Obama's health care reform law passed in 2010.

A CDC spokeswoman said that six employees had been recalled to staff a network of public health labs that monitor for food-borne outbreaks, bringing the total number of employees on that project to seven.

"The shutdown has really handcuffed these regulatory agencies and their proper regulatory role," said Democratic Congresswoman Louise Slaughter's spokesman Eric Walker.

"This is the nightmare scenario, not just with the government shutdown but this is what happens when you overuse antibiotics in livestock."

A high proportion of people have been hospitalized -- 42 percent -- and some of the salmonella strains are showing resistance to antibiotics, according to the Center for Science in the Public Interest.

The consumer advocacy group also noted that seven strains of salmonella appear to be responsible for the sicknesses.

"The number of people we know to be ill is just the tip of the iceberg," said CSPI food safety director Caroline Smith DeWaal.

"This outbreak shows that is a terrible time for government public health officials to be locked out of their offices and labs, and for government websites to go dark."

Foster Farms said in a statement that no recall is in effect and that "products are safe to consume if properly handled and cooked."

The company also said it had "instituted a number of additional food safety practices," that have been shown in the past to control salmonella.

"Salmonella is naturally occurring in poultry and can be fully eradicated if raw product is properly handled and fully cooked," said the farms' food safety chief and head veterinarian Robert O'Connor.

The FSIS warned that salmonella infections "can be life-threatening, especially to those with weak immune systems, such as infants, the elderly and persons with HIV infection or undergoing chemotherapy."

Common symptoms include diarrhea, stomach cramps and fever within eight to 72 hours of consuming infected food. Illness, including nausea, chills and headaches, may persist for up to a week.

Consumer Reports magazine said its own tests found a salmonella strain in Foster Farms chicken that matched one of those associated with the current outbreak.

"It is outrageous that Foster Farms has not issued a recall in the face of so many illnesses associated with their product," said Urvashi Rangan, toxicologist and executive director of the Consumer Reports Food Safety and Sustainability Center.

The magazine said "the outbreak is of particular concern due to its reported severity, the fact that the associated strains are resistant to multiple antibiotic drugs and the length of time that the CDC has been tracking illnesses that are now linked to Foster Farms chicken."