(Adds comments from Takata and a plaintiff's attorney)

By Bernie Woodall and Eric Beech

DETROIT/WASHINGTON, Feb 25 (Reuters) - A U.S. safety regulator on Wednesday ordered Takata Corp to preserve all air-bag inflators removed through a recall process as evidence for a federal investigation and private litigation cases.

It was the first time the National Highway Traffic Safety Administration, an agency of the U.S. Department of Transportation, has ordered a company to preserve evidence for private litigation, said Gordon Trowbridge, a spokesman for NHTSA.

The defective parts, which activate air bags in case of collision - have been linked to at least six deaths and dozens of injuries, and have resulted in several lawsuits. NHTSA claims the air bags explode with too much force, spraying metal fragments at occupants.

On Friday, NHTSA slapped a $14,000-a-day fine on Takata for failing to fully cooperate with the government's probe. Since 2008, nearly 25 million vehicles worldwide with Takata air bags have been recalled.

The directive from U.S. Secretary of Transportation Anthony Foxx also prohibits Takata from destroying or damaging any air-bag inflators except those necessary for testing. Takata is required to set aside 10 percent of recalled air-bag inflators and make them available for testing by private plaintiffs.

NHTSA wanted to ensure that all parties testing the inflators including itself, Takata, a consortium of automakers as well as private litigants could have access to them.

Several weeks ago, a federal judge in South Carolina had limited access to inflators at the request of private litigants, but opened access to parties testing them at the request of the Justice Department.

A Florida attorney who represents a client suing Takata, said a U.S. Department of Justice attorney appeared as an interested party at a recent hearing in Miami where scores of federal cases against Takata have been consolidated.

"I've never seen a federal prosecutorial authority appear as an interested party in a civil matter," said Jason Turchin, who said he has been involved in more than 4,000 cases.

A spokesman for Japan-based air-bag maker Takata, Bob Rendine, said the company will continue to work closely with NHTSA.

"We believe the outcome (Foxx's order) is in the best interest of all parties, and consistent with our commitment to the safety of the driving public. Determining the root cause of the inflator issues has been, and remains, our top priority," said Rendine.

Foxx said NHTSA would upgrade its Takata investigation to an engineering analysis, a formal step in the agency's defect investigation process.

(Reporting by Eric Beech in Washington and Bernie Woodall in Detroit; Editing by Peter Cooney, Matthew Lewis and Ken Wills)