Chrysler Group is taking the very rare step of defying the government by refusing to recall 2.7 million Jeeps that federal safety officials say are dangerous and should be recalled.

The National Highway Traffic Safety Administration sent the automaker a letter late Monday telling it to recall the vehicles and fix an alleged problem that can lead to fatal fires in rear-end crashes.

The agency, which has been investigating some of the Jeeps since August 2010, says their gas tanks are mounted behind the rear axle, so are more susceptible to damage in a rear impact. And they are too high above the road, and thus less protected in a rear crash, the agency says.