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WASHINGTON (AFP) - The United States mistakenly sent Taiwan four fuses used to trigger nuclear warheads on ballistic missiles in late 2006 and only discovered the mistake last week, the Pentagon said Tuesday.

The fuses were recovered Monday from Taiwan where they were believed to have been held in storage after being mistakenly shipped as helicopter batteries from a Defense Department logistics facility in the United States, senior Pentagon officials said.

US Defense Secretary Robert Gates has ordered an investigation into the incident and a comprehensive review of the US inventory of all nuclear related components as well as of policies and procedures, senior officials said.

\"We'll do a thorough investigation, and those who are responsible will be held accountable. The secretary is quite forceful on this,\" said Ryan Henry, principal deputy undersecretary of defense for policy.

It was the second major nuclear security breach uncovered in just over six months, following the mistaken transfer of nuclear armed cruise missiles from one US base to another aboard a B-52 bomber in September.

Investigators will seek to determine whether the fuses and nose cone assemblies in which they were housed were tampered with, as well as how the error occurred and who was responsible.

China was notified of the shipment, Henry said.

\"Our policy on Taiwan arm sales have not changed. This specific incident was an error in process only and is not indicative of our policies, which remain unchanged,\" he said.

The fuses fit in the nose cone atop the warhead of a Minuteman intercontinental ballistic missile. Although they contain no nuclear material, they are used to ignite the trigger of a Mark-12 nuclear weapon, officials said.

\"It is the electrical firing mechanism that allows the rest of the system to detonate,\" said Air Force Secretary Michael Wynne.

Wynne said the 22-inch long needle nose component does not look like a helicopter battery.

The sequence of errors began in March 2005 when the fuses were shipped as surplus articles from F.E. Warren Air Base in Wyoming to Hill Air Force Base in Utah.

Even though they were classified items, Wynne said they apparently were put in an unclassified logistics facility at Hill Air Force Base.

\"In fall 2006 is when DLA (the Defense Logistics Agency) picked them out of the warehouse in fulfillment of the FMS (foreign military sales) order and shipped them to the Republic of China, Taiwan,\" Wynne said.

Henry said \"fairly early on\" the Taiwanese communicated that what they had received was not the helicopter batteries that they had ordered.

\"We, on our side, thought we were talking about different sorts of batteries. There was an effort to resolve and to reimburse them,\" he said.

\"It wasn't until this past week that we became aware that they had something akin to a nose cone assembly, at which time elements in the field worked that for a couple of days to get resolution,\" he said.

The Pentagon became aware of the situation on Thursday after the parts numbers had been identified, he said.

Gates did not learn of the situation until he arrived at work on Friday morning, he said. President George W. Bush was notified the same day.

Henry said the nose cone assembly and fuse were classified but the technology dated from the 1960s.

But he and the other officials said that an investigation led by Navy Admiral Kirkland Donald, the director of Navy nuclear propulsion, will determine if any secrets were compromised.

Wynne said the items were understood to have been placed in storage by the Taiwanese after they received them. When the Taiwanese realized what they had, \"they notified the right authorities and started the recovery process.\"

The US Pacific Command working through US military liaison officers in Taiwan established security at the warehouse and then transported the fuses to a site under US control.

\"We have no indications from a site inspection of the item that it has in any way been tampered with, but that will be part of the further investigation,\" Henry said.

\"That's one of the reasons for bringing the intelligence services in, to be able to determine that.\"

He said the Pentagon was studying whether the mistaken shipment represented a violation of the Missile Technology Control Regime.