Fifty one labs in 17 states and three foreign countries have been mailed samples of live anthrax, Pentagon officials have said on June 3.

The announcement doubled the number of incidents in the US. A smaller number of shipments were revealed last week.

Staff members at some of the labs have been treated for anthrax exposure as a precaution, but no-one has fallen ill.

The Pentagon has maintained there is no risk to the general public.

The Centers for Disease Control and Prevention (CDC) is leading the investigation into the incidents.

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According to defense official Robert Work, the number of affected laboratories is expected to rise.

Experts in biosafety have heavily criticized the lapse and called for improved precautions.

Symptoms of anthrax exposure include skin ulcers, nausea, vomiting and fever, and can cause death if untreated.

The military has ordered all of its labs that have previously received inactive anthrax samples to test them. In addition it is advising all labs to cease working with these samples until told otherwise.

Pentagon officials say there was no sign the live samples were sent due to any deliberate action.

The samples were mailed from a Utah army facility by commercial post to laboratories over the past 10 years.

In addition to the US labs, samples were sent to facilities in Australia, Canada and South Korea.