Via Reuters

The U.S. Department of Transportation has rebuffed a call by two Democratic senators to advise owners of 2.6 million recalled General Motors cars to stop driving them until they are repaired.

In letters sent on Tuesday to Senators Edward Markey of Massachusetts and Richard Blumenthal of Connecticut, Transportation Secretary Anthony Foxx wrote that "such an action is not necessary at this time."

It could take months for GM to replace faulty ignition switches in Chevrolet Cobalts, Saturn Ions and other models that have been linked to at least 13 deaths.

Foxx said the department's National Highway Traffic Safety Administration "is satisfied that for now," until the repairs are made, the safety risk posed by the ignition switch defect is mitigated by GM's recommendation that the cars be operated with only the key in the ignition switch and no other keys or fobs attached.

The added weight is thought to cause the switches to turn from the "on" position to an "accessory" position that causes car engines to turn off. It also has resulted in air bags not deploying in crashes and power steering and brakes not operating as intended.

In a statement reacting to the agency's decision, Blumenthal and Markey said, "We remain extremely concerned that GM and NHTSA are not doing enough to convey the seriousness of this defect to owners of the affected cars, unnecessarily putting more lives at risk."

The senators have voiced concerns about GM's warning that driving over rough roads also could cause the recalled cars to turn off.

Foxx noted that GM has tested the cars over a variety of conditions, including potholes, panic stops and angled railroad crossings. He said NHTSA reviewed GM's tests "and believes the information supports GM's position that the subject vehicles are safe to operate" provided the ignition key is not attached to other items.

Two committees of Congress are investigating why it took GM more than a decade after discovering a safety problem to recall vehicles.