(Reuters) - U.S. regulators on Friday slapped Takata Corp with a $14,000 per-day fine for failing to fully cooperate with a probe of its faulty air bags and revealed that a fraction of the 17 million cars recalled because of the problems have been repaired.

The Japanese parts supplier is still struggling to come to terms with the massive recall and investigation related to the air bags, which have been prone to rupture unexpectedly, spraying shrapnel into vehicle occupants.

Defective Takata air bag inflators - which activate the devices in case of collision - have been linked to at least six deaths and dozens of injuries, and have resulted in several lawsuits.

Takata failed to comply with two orders U.S. safety regulators issued last year requiring documentation and other material for a probe into the faulty air bags, according to Transportation Secretary Anthony Foxx.

The government is accusing Takata of dumping more than 2.4 million pages of documents on the National Highway Traffic Safety Administration without any guide to or explanation of the content.

"As you are well aware, NHTSA has repeatedly engaged Takata and asked for the company's explanation of the content of the deluge of documents that it has produced thus far," NHTSA said in a letter to Takata lawyer Steven Bradbury, explaining the reason for the fine, to be levied starting on Friday.

Takata said it "strongly" disagreed with NHTSA's characterization, adding that it had been meeting regularly with NHTSA engineers to identify the cause of the issues with the inflators.

In another sign of Takata's struggles to put the issue to rest, the U.S. safety regulator also said Friday that nearly 90 percent of the vehicles recalled because of defective Takata air bags were still unrepaired as of December 31.

NHTSA has urged owners of certain vehicles from Toyota Motor Corp, Honda Motor Corp Ltd, Mazda Motor Corp, BMW AG, Nissan Motor Co Ltd, Mitsubishi Motors Corp, Subaru Co Ltd, Fiat Chrysler Automobiles, Ford Motor Co and General Motors Co to replace the air bags as soon as possible.

On Friday, Foxx also called on Congress to pass legislation requiring car rental agencies and used car dealers to fix safety defects in the vehicles they rent or sell.

Senators John Thune and Bill Nelson on Friday urged Takata to help federal regulators. "We cannot tolerate delays or limited cooperation when people’s lives are at stake," they said in a statement.