The chairman of the Senate Armed Services Committee said Tuesday the loading of nuclear-tipped missiles on a B-52 bomber at a U.S. air base last August was an unprecedented and significant security failure.

"This event is really a wake-up call," Sen. Carl Levin, D-Mich., told Air Force officers assembled for a committee hearing. "As long as the United States has nuclear weapons they must be handled with the utmost security and attention."

Several other senators echoed Levin's concerns even after the witnesses acknowledged the incident reflected poor performance.

"This was the result of a lack of attention to detail and lack of adherence to well-established Air Force guidelines, technical orders and procedures" and some two dozen personnel have been removed from their jobs, Lt. Gen. Daniel Darnell, director of Air Force operations and planning, told the committee.

Still, there was virtually no chance of a disaster because the rack, or pylon, carrying the six missiles was not powered up, Darnell said. "There was never an unsafe condition," he said.

And it was "very, very unlikely" plutonium from the warheads could have been released if the plane had crashed in an accident, said former Air Force chief Larry Welch.

Three investigations explored the incident in which the B-52 was inadvertently loaded with six nuclear-tipped cruise missiles and flown from Minot Air Force Base in North Dakota to Barksdale Air Force Base in Louisiana.

The warheads were supposed to have been removed from the missiles before the missiles were carried to the Louisiana base.

Dozens of recommendations to improve security have been adopted, Darnell said. But Levin said most of the 132 recommendations produced by the three investigations have not been implemented.

Levin, noting that each warhead had 10 times the power of the atom bomb dropped on Hiroshima, Japan, in World War II, called the loading of nuclear-tipped missiles on the bomber "a significant failure."

"While historically there have been nuclear weapons accidents, with varying degrees of severity, no breach of nuclear procedures of this magnitude has ever occurred," Levin said.

He called inattention to security, "with few exceptions," pervasive within the Defense Department.

Sen. Bill Nelson, D-Fla., blamed "sloppiness and lack of discipline," and Sen. John R. Thune, R-S.D., said, "This illustrates everyone is human, but we cannot tolerate mistakes." Thune said he was concerned that attention to nuclear safety would ebb as time passes.