"We are deeply sorry, and we are working to address this issue as quickly as we can," GM North America President Alan Batey said in a statement.
The Detroit automaker acknowledged the ignition-switch problem, which can turn the car off while it's being driven -- shutting off everything, including the power steering, anti-lock brakes and air bags -- led to at least 31 crashes and 13 front-seat fatalities.
GM's nearly decade-long investigation of the defect, which it knew about since 2004, "was not as robust as it should have been," Batey said.
The admission came as the automaker was in discussions with the U.S. Transportation Department's National Highway Traffic Safety Administration about why it took so long to recall the vehicles.
Tuesday's announcement came as GM recalled 748,000 more cars in the United States in addition to 619,000 recalled Feb. 13.
The additional models recalled are 2003-2007 Saturn Ion compact cars, 2006-2007 Chevrolet HHR retro-styled station wagons and 2006-2007 Pontiac Solstice sports cars and Saturn Sky roadsters.
These models join the 2005-2007 Chevrolet Cobalt and Pontiac G5 models recalled earlier.
A total of 1,367,146 cars in the United States are now included in the recall.
GM said it wouldn't rule out a wider recall as talks with the NHTSA continue, the Detroit Free Press reported.
A delayed response could cost GM tens of millions of dollars in civil penalties if the agency determines the automaker neglected to inform regulators, the Los Angeles Times said.
The automaker also faces the potential for multiple lawsuits or a class action, the Free Press said.
The defective ignition switches could inadvertently turn off the engine and vehicle's electrical system if the ignition key is jostled or is on a heavy key ring.
GM said Tuesday drivers of the cars should use a single key to operate their vehicles and remove all other keys and chains until after the repair is made.
The automaker said the 13 front-seat deaths in crashes occurred when the front air bags did not deploy.
For years, GM said it couldn't find the cause of the problem but said it was investigating. The automaker was aware of the faulty ignition switches as early as 2004 and issued a service bulletin for its dealers in 2005, documents it submitted to the NHTSA indicated.
The NHTSA told GM in a 2007 meeting about a crash that killed a 16-year-old Maryland girl who lost control of her 2005 Chevrolet Cobalt and slammed into a tree.
"The GM employees meeting with NHTSA on this occasion were not aware of the crash," a GM report released Tuesday said.
"Today's GM is committed to doing business differently and better," Batey said in his statement. "We will take an unflinching look at what happened and apply lessons learned here to improve going forward."