DETROIT — The nation’s top auto safety regulator says an immediate recall of all Takata air bags wouldn’t provide significant safety benefits and could exceed the government’s legal authority.

A recall of all Takata air bag inflators also would strain the network for replacement parts and increase uncertainty for consumers, Mark Rosekind, head of the National Highway Traffic Safety Administration, wrote in a letter to a legislator.

Takata’s air bag inflators use the chemical ammonium nitrate to create a small explosion that fills air bags in a crash. But they can explode with too much force, blowing apart a metal canister and hurling shrapnel at drivers and passengers. At least 10 people have died worldwide and 139 have been hurt.

So far, 14 automakers have recalled 24 million U.S. vehicles to replace the inflators. The problem has been linked to older cars with long-term exposure to high humidity. That’s why replacement parts are being targeted to areas such as the U.S. Gulf Coast.

In a letter sent Friday to Sen. Bill Nelson, D-Fla., Rosekind argued that a broader recall could divert parts from vehicles at higher risk “while achieving no identifiable safety benefit.”

On Feb. 10, Nelson asked the agency to make Takata stop manufacturing ammonium nitrate inflators and urged it to recall all Takata inflators. Nelson and other lawmakers requested a total recall of the inflators after a South Carolina man was killed by a Takata driver’s inflator when his pickup truck hit a stray cow. The driver’s inflator in the man’s Ford Ranger pickup had not been recalled, and previous tests had shown no problems with that particular type of inflator.

On Monday, Nelson said in a statement that he’s unhappy with Rosekind’s response. He’s still concerned that the government is deferring to Takata on when to phase out ammonium nitrate inflators. “If there’s one thing we have learned from our investigation, it’s that Takata can’t be trusted,” he said.

Agency spokesman Gordon Trowbridge said Monday that NHTSA has to have supporting data to seek further recalls. The agency has said that it expects the number of Takata recalls to increase.

NHTSA has given Takata until the end of 2018 to prove the inflators are safe or issue a blanket recall. The company has agreed not to sign any more contracts to sell ammonium nitrate inflators and phase out manufacturing by the end of 2018.

Last week, scientists hired by 10 automakers blamed the trouble on a combination of volatile ammonium nitrate, heat and humidity, and inflator containers that may let moisture seep in.

In addition to the 24 million recalled vehicles, at least another 50 million U.S. vehicles have Takata inflators that have not been recalled. Most of them have inflators that include a drying agent called a dessicant. Rosekind pointed out that NHTSA knows of no inflators with the drying agent that have ruptured.

Nelson also complained that NHTSA’s “piecemeal approach” to the recalls is confusing to car owners. Rosekind conceded that the recalling all the inflators would be easier to explain to consumers “but it would not serve safety and could run the risk of exceeding NHTSA’s statutory authority,” he wrote.

Auto companies, which have been hampered by a lack of replacement parts and customer indifference, are making slow progress toward getting the recall repairs done. As of Feb. 12, automakers had replaced 7.1 million inflators, about 31 percent of the air bags that have been recalled in the U.S., according to NHTSA. That’s up from about 18 percent in August of last year, the agency says.