A leading US military nuclear centre has stopped all classified work following the disappearance of vital data. The Los Alamos National Laboratory, a key centrer for nuclear weapons research, has temporarily ceased all classified work after vital data was reported missing last week from a research area, lab officials said on Thursday. Such a precaution at Los Alamos, the New Mexico birthplace of the first atomic bomb during the second World War, has not occurred in recent memory, lab officials said, highlighting the seriousness of the breach. The lab said it learned of two missing data storage devices on July 7 during an inventory check. At a news conference, the lab director and other officials declined to detail the nature of the data, citing national security concerns. "Until such time as we are confident that we are addressing this issue, then all activities with respect to classified materials have been put on hold," said Gerald Parsky, chairman of the Regents of the University of California which manages Los Alamos. "These breaches of national security will not be tolerated." **Security shortcomings*** The case of the missing disks is the latest in a series of security shortcomings at US nuclear weapons labs in recent years. Just last month a set of keys to a sensitive nuclear area at Los Alamos went missing for most of a day. "This is a big deal, but it is certainly a necessary step," Danielle Brian, director of the Project on Government Oversight, a watchdog group in Washington DC, said of the Los Alamos halt of classified work. The missing data was on two zip disk drives, she said, adding: "They need to change the way they handle classified media and move to what's called a media-less system so that there isn't the capacity for a scientist to just walk off with a disk or a zip drive." Los Alamos spokesman Kevin Roark said "fewer than 20" staffers have had their lab access suspended pending the results of the inquiry. **PHOTO CAPTION*** Secret work at key US nuclear weapons facility, Los Alamos National Laboratory, was halted after a fresh security breach in which classified data went missing. (AFP)