The US Defense Department announced Tuesday that the United States mistakenly shipped to Taiwan four electrical fuses designed for use on intercontinental ballistic missiles but has since recovered them. The error is particularly disturbing, officials said, because of its indirect link to nuclear weaponry and because of the sensitivity of US arms sales to Taiwan, which China regularly denounces as provocative. The Defense Department said an investigation of the incident is under way. At a news conference, Air Force Secretary Michael Wynne said the mis-shipped items were four electrical fuses for nose cone assemblies for ICBMs. He also said they were delivered to Taiwan in 2006 and had been sent instead of helicopter batteries that had been ordered by Taiwan. It is the second nuclear-related mistake involving the Air Force in recent months. Last August an Air Force B-52 bomber was mistakenly armed with six nuclear-tipped cruise missiles and flown across several states. At the time, the pilot and crew were unaware they had nuclear arms aboard.