The Defense Department mistakenly shipped secret nuclear missile fuses to Taiwan more than 18 months ago and did not learn that the items were missing until late last week, Pentagon officials acknowledged yesterday, deepening concerns about the security of the U.S. nuclear arsenal.

Officials with the Defense Logistics Agency (DLA) sent four nose-cone fuse assemblies to Taiwan in August 2006 instead of four replacement battery packs for use in Taiwan's fleet of UH-1 Huey helicopters. The fuses help trigger nuclear warheads on Minuteman intercontinental ballistic missiles as they near their point of impact. It was unclear yesterday how the two very different items were mixed up at a warehouse at Hill Air Force Base in Utah and how they were shipped out of the country without notice.

Defense Secretary Robert M. Gates immediately ordered an investigation, the second such probe in the past year to examine serious lapses in the care of U.S. nuclear weapons and accessories. Gates learned of the erroneous shipment on Friday and informed President Bush, but officials waited until yesterday -- after Saturday's elections in Taiwan -- to disclose the incident. Pentagon and State Department officials have conferred with Taiwanese and Chinese diplomats over the past three days.

"In an organization as large as DOD, the largest and most complex in the world, there will be mistakes," said Ryan Henry, principal deputy undersecretary of defense for policy, speaking at the Pentagon yesterday. "But they cannot be tolerated in the arena of strategic systems, whether they are nuclear or only associated equipment, as was in this case." Gates found the incident "disconcerting," he added.

In August, the Air Force lost track of six nuclear warheads for 36 hours when they were inadvertently flown on a B-52 bomber between bases in North Dakota and Louisiana. The incident exposed security flaws and raised similar questions about the safety of U.S. nuclear weapons.

Senior defense officials said it was almost certainly human error that led to the nose cones being shipped, and Air Force officials were concerned the classified items were placed in an unclassified area of a DLA warehouse and not properly tracked. Quarterly inventory checks over the past 18 months did not show the nose cones were missing.

A DLA spokesman did not respond to questions about the incident. A spokeswoman for the Taipei Economic and Cultural Representative Office, Taiwan's principal representative office in the United States, declined to comment.

Missile defense experts said the United States may have violated nuclear nonproliferation agreements and U.S. export laws by sending the items to Taiwan. Such treaties and regimes are designed to prevent the transfer of nuclear technologies between countries, and sensitive nuclear missile parts are among the most regulated items.

"This is a case of horrifying mismanagement of the inventory at this location," said Leonard S. Spector, deputy director of the James Martin Center for Nonproliferation Studies. "But it does seem more like mismanagement rather than a nefarious scheme to get them to Taiwan."

Since 2003, the Air Force had made 139 separate transfers of classified parts between F.E. Warren Air Force Base in Wyoming and the base in Utah -- mainly to store excess parts in a DLA warehouse -- and only the March 2005 transfer of four nose cones was misplaced, two defense officials said. How that oversight occurred will be at the center of the investigation.

Taiwan received four drum-shaped packages from the United States in August 2006 and placed them, unopened, into storage. Taiwanese officials realized only recently that the packages contained the nose cones when they went looking for the helicopter batteries, according to U.S. defense officials.

In trying to arrange reimbursement for the missing battery packs, U.S. officials determined that the drums contained classified material, quickly secured the items and returned them to the United States.