A total of 18 laboratories may have accidentally received live samples of anthrax shipped out by the American military, US authorities said Thursday.

But as of yet, only one of the facilities -- in the US state of Maryland -- was confirmed to have received a live sample, the Pentagon and Centers for Disease Control and Prevention said.

On Friday, a private firm in Maryland notified authorities that its sample was still active, setting off an urgent review of all material sent out to other labs.

The 17 other laboratories, including one at the US Osan Air Force Base in South Korea, received samples from the same batch and are therefore likely to also be live, the Pentagon and the Us Centers for Disease Control (CDC) said.

The samples "are being carefully transferred to the CDC" in Atlanta, Jason McDonald, a spokesman for the agency, told AFP.

"They are starting to arrive," he said, but cautioned that the samples need five more days to grow before officials can determine if they contain live anthrax.

The 18 labs were shipped the anthrax from Dugway Proving Grounds, a Defense Department laboratory in Utah, where the samples were irradiated and supposedly rendered "dead."

Four lab employees in the states of Texas, Delaware and Wisconsin had been put on antibiotics as a precaution, McDonald said.

Meanwhile, 22 military and civilian personnel were also placed under preventative treatment at Osan Air Force Base, according to the Pentagon.

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