The Los Alamos National Laboratory in the United States has temporarily ceased all classified work after vital data was reported missing last week from a research area.
Such a precaution at Los Alamos, the New Mexico birthplace of the first atomic bomb during World War Two and a key US centre for nuclear weapons research, has not occurred in recent memory, lab officials said.
The lab said it learned of two missing data storage disks on July 7th 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," Mr Gerald Parsky, chairman of the Regents of the University of California, which manages Los Alamos, said.
The case of the missing disks is the latest in a series of embarrassing episodes at US nuclear weapons labs in recent years that have raised concerns about lax security standards.
Officials said however that they had no indication that the sensitive data had been taken outside of the well-secured Los Alamos facility which is spread over nearly 40 square miles.
Four years ago a Taiwanese-American scientist at Los Alamos, Mr Wen Ho Lee, was accused of stealing secrets from the nuclear research centre. He later pleaded guilty to one count of downloading nuclear weapons design secrets to a non-secure computer after the government's case against him collapsed.