WASHINGTON (CNN) -- Defense Secretary Robert Gates has formally ordered the Air Force, Navy and Defense Logistics Agency to conduct an inventory of all U.S. nuclear weapons and nuclear weapon-related materials to make sure all items are accounted for, according to a Pentagon memo released Thursday.

Defense Secretary Robert Gates orders an inventory of all U.S. nuclear weapons and related materials.

The order comes in the wake of the discovery last week that four nuclear warhead fuses were accidentally shipped to Taiwan in 2006.

Gates' memo, issued Wednesday, calls for all items to be accounted for by serial number.

Pentagon officials said at a news conference Tuesday that Gates would call for the review in addition to a full investigation into how the shipment to Taiwan from a Defense Logistics Agency warehouse happened 18 months ago.

The inventory review, which will involve thousands of items, is due to Gates in 60 days. Pentagon officials said the request was ordered, in part, because this latest incident comes after the August 2007 accidental flight of six nuclear-tipped cruise missiles on a B-52 bomber across the country.

"At a minimum, your report should include the results of the inventory and your personal assessment of the adequacy of your respective department or agency's positive inventory control policies and procedures," Gates said in the memo.

Four officers --- including three colonels -- were relieved of duty last year after a B-52 bomber mistakenly carried six nuclear warheads from North Dakota to Louisiana, the Air Force said.

A six-week investigation uncovered a "lackadaisical" attention to detail in day-to-day operations at the bases involved in the incident, an Air Force report said. E-mail to a friend

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