Media playback is unsupported on your device Media caption Biological Defence Commander Franca Jones demonstrated how Anthrax is shipped

More than 50 labs in 17 US states and three foreign countries have been mailed samples of live anthrax, Pentagon officials have said.

The announcement on Wednesday 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.

US defence official Robert Work told reporters that the number of affected laboratories is expected to rise.

Image copyright AP Image caption Workers must wear protective clothing when dealing with anthrax

Experts in biosafety have heavily criticised 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.

What is anthrax?

Anthrax bacteria live primarily in inactive spores and are found naturally in the soil, but people can ingest or inhale spores, which can make the anthrax active.

Not all will be sickened when exposed to anthrax but left untreated, anthrax illness can turn very serious or lead to death.

A blood culture test is used to confirm anthrax exposure.

Anthrax entered the US national consciousness in 2001, when shortly after the 9/11 attacks, letters containing powdered anthrax arrived at news organisations and the offices of US senators. Twenty-two people were sickened and of those, five people died.

Sources: FBI Anthrax investigation; CDC, San Francisco Department of Public Health