National groups representing fire and emergency rescue workers accused the Ford Motor Company yesterday of "foot-dragging" in the automaker's efforts to correct fuel tank problems on ambulances that have caught fire across the country.

A Ford spokeswoman, responding to the charges, said at a news conference that a project started last month to modify the ambulances is taking time because problems with the Ford E-350 chassis are complicated by about 45 ambulance manufacturers who each construct the ambulances differently.

The International Association of Fire Chiefs, the American Ambulance Association, the National Association of State EMS Directors and the Center for Auto Safety also repeated a request yesterday that the National Highway Traffic Safety Administration initiate a recall of as many as 20,000 ambulances built on the 1983-86 Ford E-350 chassis.

Meanwhile, Virginia Attorney General Mary Sue Terry said yesterday she is preparing a lawsuit against Ford to be filed if the automaker doesn't solve the ambulance problems.

"We've been patient, and we've had personnel injured because of our patience," said Garry L. Briese, executive director of the International Association of Fire Chiefs.

Since the middle of May, according to the Center for Auto Safety, nine ambulances built on the Ford E-350 chassis have caught fire. A Charles County ambulance caught fire and was destroyed on Monday outside of the Southern Maryland Hospital Center. Officials in the county said work was done two weeks ago under the supervision of a Ford engineer from Michigan to modify the $60,000 ambulance.

"Ford's foot-dragging has been aggravated by the fact that the {Charles County} ambulance that caught fire had been {modified} by Ford two weeks ago," said Robert Forbuss, president of the American Ambulance Association.

Ford and ambulance companies that modify the Ford chassis for use as emergency medical vehicles announced a plan last month to correct fuel tank problems that have been blamed for 15 ambulance fires, 14 injuries and more than 200 incidents in which boiling gasoline spurted out of the fuel tank when the gasoline cap was removed, according to the Center for Auto Safety, a private consumer agency.

The Ford plan, initially scheduled to involve vehicles in Virginia, Maryland and the District of Columbia, resulted from earlier pressure from Terry, who threatened to sue the country's second-largest automaker after two ambulances built on the Ford chassis caught fire in March.

Helen O. Petrauskas, Ford vice president for environmental and safety engineering, said that the modification project has been extended to Ohio and Kentucky. Ford is notifying public and private ambulance operators across the country to tell them that the company will inspect and modify their vehicles built on the E-350 chassis at designated Ford dealerships.

Some of the fire and emergency medical service leaders questioned yesterday whether Ford's modification kits, which are designed to eliminate a tendency for the vehicles to overheat, are adequate to correct the problem. They cited the case of the Charles County ambulance, and said that five other ambulances that have been modified in Virginia have developed other mechanical problems.

A National Highway Transportation Safety Administration spokesman said that he could not comment about an ongoing investigation. The spokesman said that the agency's engineers have been conducting a formal investigation -- the highest level -- of ambulances built on the E-350 chassis since April 27.

The government agency has not forced a vehicle recall since 1983, an agency spokesman said.