* Senators push Takata to expand recall across U.S.

* Takata admits it may have a parts shortage

* Federal regulator asks for increased penalties for industry (Adds comment from Subaru spokesman in paragraph 20)

By Eric Beech and Ben Klayman

WASHINGTON/DETROIT, Nov 20 (Reuters) - An executive from Japan's Takata Corp told U.S. senators on Thursday that the supplier is urgently trying to ramp up replacement parts for millions of vehicles equipped with potentially deadly air bags, but said it may not be able to move quickly enough.

The U.S. auto safety regulator also warned of the risks of moving to a nationwide recall, as senators have urged, saying such a move could divert replacement parts from humid regions where the defective air bags are more likely to rupture upon deployment, shooting metal shards into cars.

At least five fatalities have been linked to the defect so far, mostly in the United States.

The hearing by the U.S. Senate Commerce Committee exposed several blind spots of regulators and the auto industry about the scope and urgency of the air bags' dangers.

About 16 million cars with Takata air bags have been recalled worldwide, with more than 10 million of those in the United States. But regulators and Takata - which supplies one in five air bags globally - have yet to pinpoint why the parts are at risk.

Hiroshi Shimizu, Takata's senior vice president for global quality assurance, acknowledged on Thursday that even if the company ramps up production of replacement kits beyond the current pace of 300,000 a month, it may still not have enough parts.

"Even if we increase to 450,000, maybe still that's not speedy enough," he said.

David Friedman, deputy administrator of the U.S. National Highway Traffic Safety Administration, told the committee his agency is in touch with two other suppliers to determine whether they are able to make replacement parts.

Friedman came under fire for the NHTSA allowing automakers to send out notices of "safety campaigns" rather than formal recalls, leaving customers confused over the severity of the problem.

Friedman said his agency would have more control over automakers if Congress passed legislation raising the maximum allowable fine to punish uncooperative automakers, which is currently capped at $35 million.

"We now have a new problem that we are addressing, which is in effect a live hand-grenade in front of a driver and a passenger," said Democratic Senator Bill Nelson, who chaired the hearing, the first to examine the deadly air bag saga that came to light in 2008 and has escalated in recent months.

The hearing held high stakes for Takata, which is facing a criminal probe into the scandal, more than 20 class action lawsuits and an NHTSA probe.

When pressed by Republican Senator Dean Heller for Takata to take "full responsibility" for five deaths linked to the air bags, Shimizu consulted a colleague multiple times.

He answered that two of the five fatalities were still under investigation, but acknowledged "anomalies" with Takata air bag parts involved in some fatal accidents. Shimizu said in his prepared comments that Takata was "deeply sorry and anguished about each of the reported instances."

Rick Schostek, Honda's North American executive vice president, was also asked about the slow rollout of recalls that started in 2008. It was only this month that the automaker turned its "safety improvement campaign" into a formal recall.

"I think we acted with urgency, but do I think we could have moved faster in some respects? I absolutely do," Schostek said.

'ROOT CAUSES'

Shimizu said the company believes the "root causes" of the air bag inflator ruptures are a combination of the age of the inflator, persistent exposure to high humidity, and problems in production.

The recalls so far have been focused on humid areas. That approach was questioned at a news conference before the hearing, when two U.S. senators linked the air bag defect to a 2003 death in Arizona, which is not considered a humid area.

Charlene Weaver, 24, died in a Takata air bag-related accident while she was a passenger in a 2004 Subaru Impeza in Arizona, her sister, Kim Kopf, said. That model car was not recalled until July of this year.

A Subaru spokesman said the company was just learning of the incident but that, in general, there are many factors in any accident. He added the company has no information to suggest the fatality could be linked to the defect that is the focus of the current recall.

The senators raised the possibility of Weaver's death as the sixth fatality linked to Takata air bags and the first reported outside of Honda vehicles.

Democratic Senator Ed Markey said the incident shows the need for a nationwide recall. "Every single one of these Takata air bags could be a ticking time bomb," he said.

NHTSA on Tuesday called on Takata and five automakers to expand their regional recalls of driver-side air bags to cover the entire United States, as senators have urged. But NHTSA's Friedman acknowledged on Thursday that such a move carries risks.

"At this point, a national recall of all Takata air bags would divert replacement air bags from areas where they are clearly needed, putting lives at risk," he said.

(Additional reporting by Bernie Woodall in Detroit, Doina Chiacu and Elvina Nawaguna in Washington; Writing by Julia Edwards; Editing by Karey Van Hall, Matthew Lewis and Ken Wills)