WASHINGTON (Reuters) - The U.S. military mistakenly shipped four fuses for nuclear missiles to Taiwan in 2006 and never caught the error, the Pentagon said on Tuesday, acknowledging an incident likely to rile China.

The military was supposed to ship helicopter batteries to Taiwan, but instead sent fuses used as part of the trigger mechanism on Minuteman missiles. Taiwan returned the parts to U.S. custody last week.

No nuclear material was shipped to Taiwan, Pentagon officials said.

The problem went unnoticed until Taiwan realized it did not have the helicopter batteries it ordered and reported the issue to the United States, U.S. officials said.

The United States has notified China, which claims the self-ruled island as its own, and is modernizing its military to close the technology gap with Taiwan’s mainly U.S. weapons. The two sides have been run separately since 1949, and Beijing has never renounced the use of force to bring the island under its control.

“The secretary of defense is taking this very seriously. We are all taking this very seriously,” Air Force Secretary Michael Wynne said.

“Though this was not and could not be construed as nuclear material ... I would tell you that we are very concerned about it,” he told reporters at the Pentagon.

The erroneous fuse shipment was the Pentagon’s second embarrassing misplacement of nuclear or nuclear-related equipment announced in recent months. An Air Force bomber mistakenly carried nuclear warheads over the United States in August 2007.

Taiwanese helicopters during an anti-airborne exercise in central Taiwan, July 27, 2005. The U.S. military mistakenly shipped four fuses for nuclear missiles to Taiwan in 2006 and never caught the error, the Pentagon said on Tuesday. The military was supposed to ship helicopter batteries, but instead sent fuses used as part of the trigger mechanism on Minuteman missiles. REUTERS/Richard Chung

INVENTORY

The Defense Department has ordered the Navy and Air Force to take inventory of all nuclear and nuclear-associated equipment and material and U.S. Defense Secretary Robert Gates has ordered an investigation into the fuse incident, said Ryan Henry, principle deputy undersecretary of defense for policy.

Gates wants initial results from the investigation by April 15 and a full report within 60 days.

The United States may have violated international law or international arms agreements with the shipment.

“That’s under analysis now,” Henry said. “If there was something that was amiss, it clearly was not intentional.”

What led to the fuse shipment is still unclear, Henry and Wynne said. The Pentagon also does not yet know who was responsible.

The fuses, which send an electronic signal to the device that starts the nuclear weapon’s trigger process, are among a class of sensitive equipment that must be accounted for on a quarterly basis.

Based on the information now known, the four fuses, which do not resemble helicopter batteries, were wrongly placed in an unclassified storage area. They were then shipped in late 2006 to Taiwan, which placed them in storage.

When Taiwan realized it had received the incorrect shipment, it notified the U.S. military. Wynne would not say when Taiwan first reported the problem.

But U.S. military officials did not understand the nature of the problem until last week.

“We on our side thought we were talking about different sorts of batteries. There was an effort to resolve and reimburse them. It wasn’t until this past week that we became aware that they had something akin to a nose-cone assembly,” Henry said.

Gates and President George W. Bush were told on Friday, the day the fuses were returned to U.S. custody, officials said.

White House spokeswoman Dana Perino said Bush still had confidence in Air Force leadership.

Henry said there was no indication Taiwan tampered with the fuses before returning them.