DETROIT - At least 400,000 of the 4 million replacement inflators for defective Takata air bags will need to be replaced again in U.S. vehicles, sources at the Japanese safety equipment maker and the U.S. safety watchdog told Reuters.

Another 500,000 of those parts appear to be safe, according to U.S. safety regulators, leaving the safety of more than 3 million replacement parts in question.

But no one seems to be able to tell owners with any certainty just how many vehicles may still contain defective original or replacement parts.

Takata Corp in mid-May told the National Highway Traffic Safety Administration that nearly 34 million air bag inflators in the United States have the potential to rupture, sending spraying metal fragments inside vehicles. At least six deaths and hundreds of injuries are linked to ruptured inflators.

NHTSA previously had identified problems with about 17 million Takata inflators.

But it is not clear how many vehicles are equipped with defective inflators, although the number appears to be far fewer than 34 million.

An unspecified number of vehicles have Takata air bags on both driver and passenger sides - and in some cases, both may be defective. In other cases, repair parts installed in those vehicles also may be defective.

Since Takata's May 18 disclosure, eight of 10 automakers that use Takata air bags have either expanded earlier recalls or initiated a new recall, and one other has said potentially defective parts are covered by previous recalls.

But the companies collectively have added only 3.3 million vehicles to the recall roster since May 18. Neither Takata nor NHTSA can explain the disparity.

The confusion extends to how many vehicles with replacement parts will need to go back to the dealer for more repairs. At least one automaker partly addressed the issue on Thursday.

In a recall expansion notice posted on Thursday by NHTSA, Honda Motor Co, one of Takata's largest air bag customers, said an unspecified number of owners who had replacement parts installed since September 12, 2014, "received an inflator of a different design, and therefore are not included in this recall."

Honda told Reuters that it has installed 1.2 million driver-side replacement air bags since Sept. 12. The kits were assembled by Takata, using inflators "of a different, contemporary design," made by Takata and other suppliers, the carmaker said.

A NHTSA official told Reuters on Wednesday that it was up to Takata and the carmakers to "demonstrate to us that the remedy parts are safe for the life of the vehicle." A priority now is to determine which of the replacement parts "are suspect and need to be replaced" again, the NHTSA official said.

Earlier in the week, when asked how consumers would know whether a replacement part would last for the life of the vehicle, NHTSA Administrator Mark Rosekind acknowledged that the issue was "confusing" and told lawmakers that owners should check with car dealers' service departments.

Takata and its contract suppliers have been gradually ramping up shipments of replacement parts to automakers.

An estimated 400,000 of the replacement driver-side inflators use a potentially defective propellant wafer that is shaped like a batwing. "Those will have to be replaced again," the Takata source said. That estimate was confirmed by the NHTSA source.

This year, about 500,000 replacement parts were made for Takata by outside suppliers, including TRW Automotive Inc and Autoliv Inc, according to the Takata source. The replacement parts from TRW and Autoliv use a different chemical from the Takata-made inflators.

"I don’t think we have any reason to suspect any problems with products from other suppliers," the NHTSA source said.

By year end, Takata expects to provide at least 1 million inflators a month, of which about 700,000 will be made by TRW, Autoliv and others.