Deputy US Energy Secretary Kyle McSlarrow and other officials toured the top secret laboratory in New Mexico at the start of a comprehensive review of security problems that shut down all work at the key installation.

The delegation that included members of the US House of Representatives Energy and Commerce Committee met staffers and received in-depth briefings on the lapses that could have led to the spilling of US nuclear secrets, officials said.

"They spent several hours touring the area involving the recent incident of the unsafe electronic media," plant spokesman Jim Danneskiold told AFP.

The visit came after all secret operations and virtually all other work were idled at the plant where the atomic bomb was born in 1945 after a several security violations that officials have branded as "unacceptable."

The unprecedented step was taken after classified computer storage discs possibly containing nuclear secrets were reported missing July 7 and after classified information was sent out in unclassified e-mails that hackers could have gained access to, officials said.

Theories of how the storage discs disappeared ranged from innocent misplacement of the disks to nuclear technology theft.

"All employees are now involved in security and safety re-education that includes looking at their work area, work controls and others aspects of security," Danneskiold said.

"Managers have been told they need to meet face to face with their employees and make sure that their training is up to date before they can re-start work and report any potential safety and security weaknesses."

The spokesman said the breach of the site's complex secure e-mail protocols had been reported to the National Nuclear Security Administration, which is in charge of reassessing security at the troubled Los Alamos plant, "to prevent any significant risk to national security."

Peter Nanos, the director of Los Alamos National Laboratory where US nuclear weapons are designed and tested, Thursday ordered a halt to secret work at the plant and then stopped virtually all operations a day later.

Around 7,800 people are employed at the site by the University of California, which runs Los Alamos, while up to 13,000 people can be present at the laboratory during the busy summer months, according to Danneskiold.

But while operations have been halted, he insisted that the stand-down posed no danger to national security as key weapons personnel were still carrying out their duties while reviewing security.

"Key deliverables for weapons certifications that Washington requires are continuing on one hand, while on the other staff make sure that they review their safety and security thoroughly," he said.

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