The US military on Thursday ordered a production freeze and safety review at all defense department labs that handle biological agents and toxins including anthrax.

It follows the discovery of evidence of anthrax contamination at the Dugway Proving Ground research center in the Utah desert, as well as shoddy record keeping at two other facilities, the Pentagon said.

It is the latest twist in a scandal that has rumbled on through the summer, after the military revealed in May that it had accidentally sent out samples of live anthrax from Dugway.

As part of the ongoing investigation at Dugway, officials there found evidence of contamination in secure areas located outside the primary containment area.

There was no indication of employees being exposed to hazardous materials and the public was not at any risk, the Pentagon said.

Live anthrax spores were inadvertently shipped from Dugway to all 50 states and nine foreign countries during a decade-long program to provide deactivated samples of the deadly pathogen to private and military labs for research and testing.

The Pentagon had already issued a moratorium on the shipping of inactivated anthrax from its facilities.

Thursday's order from Army Secretary John McHugh expands the moratorium to include production freezes of biological agents at four Defense Department labs including Dugway.

"These measures will remain in place pending completion of the ongoing review and investigation until the Army determines it is appropriate to resume operations," the Pentagon said in a statement.