The U.S. government today called for a major recall of nearly 3 million Chrysler sport utility vehicles based on what its experts say is a deadly fire hazard that has left an estimated 51 people dead.

"This will be the biggest recall [in terms of impact] since the Ford Pinto," said Clarence Ditlow, executive director of the Center for Auto Safety, which requested a U.S. probe into the SUVs in 2010.

The automaker is refusing, however, saying that its vehicles are safe and not defective.

The government says that initial findings show a safety defect in older model Jeeps - the Jeep Grand Cherokee from 1993 to 2004, and the Jeep Liberty 2002 to 2007.

The government also says the fuel tanks are vulnerable to rupture in rear-end collisions.

Photo credit: Jenelle Embrey

Crash tests done by engineering students at a government test facility showed fuel spilling from the Grand Cherokee. Chrysler contends the tests were three times more severe than government standards.

Today the National Highway Traffic Safety Administration called on Chrysler to recall older model Jeep Grand Cherokees as well as Jeep Liberties, saying "the defect presents an unreasonable risk … because people [in the vehicles] and in striking vehicles have burned to death in rear impact crashes."

According to the government, the problem is the fuel tank location, just behind the rear axle and slightly below the bumper. In later models of the Grand Cherokee and Liberty - in 2005 and '07, respectively - the tank was moved but Chrysler said it was for design reasons.

Chrysler insists the government's three-year-long investigation is based on an "incomplete analysis."

In a statement to ABC News, the automaker said: "The company does not agree with NHTSA's conclusions and does not intend to recall the vehicles cited in the investigation. … Chrysler Group stands behind the quality and safety of its vehicles. It conducts voluntary recalls when they are warranted, and in most cases, before any notice or investigation request from NHTSA. "

Jenelle Embrey, who's waged a campaign against these late-model Jeeps since October, was infuriated by Chrysler's reaction to the recall.

In October, she was in a horrific rear-end crash on a Virginia highway. A tractor-trailer slammed into a Jeep Grand Cherokee stopped in traffic and the Jeep rammed into the car carrying Embrey and her father. The two of them walked away but a small fire started growing in the back of the Jeep.

Embrey's father ran over to the vehicle. She said the doors were jammed so he busted the window with his hands.

He pulled out Zack Santor, 18, but the blaze moved so fast that he was unable to rescue Santor's badly injured mother or Santor's friend.

"Something has to be done," Embrey said today. "It can't be done soon enough."