The US army has accidentally shipped live anthrax samples to numerous laboratories inside the US and to South Korea.

The dangerous mistake was probably the result of a failure in the irradiation process designed to render the bacteria harmless, the army’s top general told the media.

In its deactivated form, the anthrax would have been used for research purposes. Instead, the potentially live samples have triggered a public health scare, as spores can survive in open air for years.

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“The best I can tell, it was not human error,” US Army chief of staff General Ray Odierno told media on Thursday.

Of the eighteen laboratories at risk in the US, at least one in the state of Maryland definitely received the dangerous live form of the bacteria, the Pentagon has confirmed.

It will take a further five days to determine if the other labs, one of which is located in South Korea, also received live anthrax.

The samples were reportedly sent using private courier service FedEx, and could have come into contact with other regular mail if packaging ruptured.

Four lab employees in the states of Texas, Delaware and Wisconsin had been put on antibiotics as a precaution, and 22 military and civilian personnel have been placed under preventative treatment at Osan Air Force Base, a Pentagon spokesman said.

Irradiation of the batch was carried out in March 2014 and the samples were not sent to private laboratories until April 2015.

Anthrax is usually lethal if left untreated, but can be cured with antibiotics.

-with AAP.