(Reuters) - Japanese auto parts supplier Takata Corp on Wednesday acknowledged that it still does not understand what is causing air bag explosions that have hurtled metal shards into vehicles but insisted that calls for a broader recall to remedy the problem were misguided.

Takata is under pressure from U.S. lawmakers and safety regulators to expand to all 50 states a recall of driver-side air bags, but the company has said data does not support such a move and it could divert replacement parts from the most-needed areas.

So far, Takata and automakers have focused attention on regions with high humidity believed to make aging air bag propellants more volatile.

"Congressman, we don't identify the root cause yet," Hiroshi Shimizu, a Takata safety executive who gave his testimony with help from an interpreter told Representative John Sarbanes. "But we are of the strong opinion that (there) is a factor contributing to this defect: which is high humidity, temperature and the life of the product."

The exchange came at a House Energy and Commerce subcommittee hearing on the Takata air bag problems which took place a day after Takata told the National Highway Traffic Safety Administration that current data does not support the need for such an expanded recall.

Still the stance of Takata, which is at the center of a global safety scandal that has involved the recall of more than 16 million cars worldwide and been linked to at least five fatalities, may be increasingly at odds with its customers.

A Honda Motor Co executive said on Wednesday that the automaker will expand the driver-side air bag recall to a national campaign, but added priority for the replacement parts should still be given to regions with higher humidity.

FILLING THE GAP?

Takata has said its monthly output of air bags will touch at least 450,000 in January - a pace that would not allow it to meet a 4.1 million unit demand until summer 2015. Takata also said an expansion of the driver-side air bag recall would add more than 8 million more vehicles to the mix.

Takata's rivals could fill some of the gap. Sweden's Autoliv Inc said on Wednesday it will begin making replacement inflators for Honda in about six months, and Honda said it may also turn to Japan's Daicel Corp for such parts.

Shimizu on Wednesday acknowledged Takata was not building the replacement parts fast enough.

Takata also said it was surprised by NHTSA's request for a broader recall because a defect investigation has not been concluded.

NHTSA late on Tuesday called the Japanese parts maker's refusal to back a national recall "disappointing," a sentiment echoed by lawmakers during Wednesday's hearing.

NHTSA has said it will likely take months to clear the bureaucratic hurdles needed to demand a nationwide recall.

Separately, Mark Rosekind, who was nominated last month to take over the safety regulator, said at his Senate confirmation hearing on Wednesday that the agency needs to move faster when addressing safety crises.

LAWMAKERS' WRATH

"I'm very concerned like all of you have been with the slowness across all of the recalls," said Rosekind, who is a member of the National Transportation Safety Board, the agency that investigates major transportation accidents.

While the safety regulator's aggressiveness has been faulted, Takata, and its reluctance to expand beyond the driver's side regional recall, was the main target of House lawmakers' wrath.

"Does that make sense to you?" Rep. Henry Waxman asked the executives of a recall campaign that would warn Florida drivers about air bag dangers but not consumers in neighboring Georgia.

"If you are in Florida, just below the line, you have to go in and get a replacement?"

Several lawmakers also questioned Takata's use of ammonium nitrate as a propellant or explosive to inflate the company's air bags but Shimizu said the compound was stable and safe.

NHTSA Deputy Administrator David Friedman said on Wednesday his agency would hire outside experts within a week to help supplement testing and learn what is causing the air bags to explode with too much force, including looking at whether ammonium nitrate was a factor.

Takata, whose air bags supply roughly a fifth of all cars on the road, and automakers who are its customers have struggled to pinpoint the exact cause of the defect.

Humid weather may provoke a dangerous air bag discharge, Shimizu said, explaining the company's rational for endorsing a recall in Puerto Rico, the Virgin Islands and Florida.

Some lawmakers questioned why NHTSA wasn't pushing for a nationwide recall of certain Takata passenger-side air bags as well. Friedman reiterated the agency's stance that the data does not support such a move at this time.

Lawmakers said Takata's answers were unsatisfactory and gave the public little confidence that it has a handle on the scope of the safety crisis.

"Complexity is not an excuse for incompetence," said Republican Representative Fred Upton of Michigan. "What should I say to the mom in Michigan who asks me if she and her family are safe behind the wheel?"