US Nuclear Missiles Go Offline, Hardware Glitch Cited

* Missiles offline for about 45 minutes

* No outside actors involved, hardware glitch eyed

* Same problem happened at least twice in past

By Phil Stewart

WASHINGTON (Reuters) - President Barack Obama has been briefed about a suspected hardware glitch that took 50 of America's 450 nuclear inter-continental missiles (ICBMs) off-line for about 45 minutes, U.S. officials said Tuesday.

The official, speaking on condition of anonymity, told Reuters this appeared to be very similar to two previous incidents, both of which occurred more than a decade ago.

The latest incident took place Saturday morning at F.E. Warren Air Force Base in Wyoming, one of three bases hosting the U.S. arsenal of Minuteman III ICBMs. The other two are in the U.S. states of North Dakota and Montana.

"We think it has happened before. How often or whether this is the biggest ever, I don't know. That's why we're looking into it," the official said.

No computer virus or outside actors were believed to be behind the glitch and the missiles could have been launched, if necessary, by activating back-up systems, the official added.

"We have no indication that this was intentional," the official said, dismissing the chances it was the result of a cyber attack or sabotage. "There's no indication of anything like that."

One of the five launch control centers responsible for the 50 missiles operated by the 319th Missile Squadron apparently tripped up communications signals between the missiles and the centers.

The official blamed a hardware problem investigators have broadly identified but are still trying to pin down.

It took 45 minutes for the first launch control center to re-initiate contact with the missiles, and another 15 minutes for the other four centers to do the same.

'HARDWARE ANOMALY'

"It was a hardware anomaly in the communications system that caused these signals to step on each other," the official added.

Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists in Washington, said the incident could have implications for war planners.

"That really downgrades what you can expect the (nuclear) force to be able to perform in a crisis," Kristensen said, adding that, backup systems or not, the incident would affect the reliability statistics war planners use.

Still, the official described the incident as a glitch that could be resolved without affecting overall reliability.

A second U.S. official, also speaking on background, said Obama was briefed on the incident, but did not offer details, including on when the president was briefed.

The first U.S. official said previous reports showed the same problem happened "twice in the past that we know of at different missile fields at different bases."

Asked about the number of missiles affected in those cases, the official said: "It was probably about the same magnitude." But he declined to offer specifics.

Kristensen said he had not heard of an incident like this happening before.

Obama has outlined plans to spend $80 billion on the aging U.S. nuclear stockpile, which would come on top of more than $100 million in additional investments in nuclear delivery systems. (Additional reporting by Ross Colvin; Editing by Jerry Norton)