DETROIT (AP) — The U.S. government's highway safety agency has rejected a request from Ford and Mazda to avoid recalling about 3 million vehicles with potentially dangerous Takata air bag inflators.

The National Highway Traffic Safety Administration said Tuesday the ammonium nitrate propellant used to inflate the air bags is showing signs of decay and poses a safety risk.

The companies now have 30 days to give the agency a schedule to notify owners and begin the recalls.

Vehicles included are the 2007 through 2011 Ford Ranger; the 2006 through 2012 Ford Fusion, Lincoln Zephyr and Lincoln MKZ; the 2006 through 2011 Mercury Milan; the 2007 through 2010 Ford Edge and Lincoln MKX. Also covered are Mazda's 2007 through 2009 B-Series small pickups.

Takata used ammonium nitrate to create a small explosion to inflate air bags in a crash. But the chemical can deteriorate over time when exposed to high heat and humidity and can burn too fast, blowing apart a metal canister and hurling shrapnel into drivers and passengers.

The air bags have caused at least 27 deaths worldwide, including 18 in the U.S. About 400 have been injured.

The problem caused the largest series of auto recalls in U.S. history, with at least 67 million inflators recalled by 19 automakers. A court-appointed monitor reports that as of early January, 50 million had been repaired or were otherwise accounted for. About 100 million inflators have been recalled worldwide.

All of the vehicles covered by the recall announced Tuesday have a moisture-absorbing chemical in the inflators that Ford and Mazda said made them safe. But the safety agency disagreed and denied the automakers' petition to avoid the recall.