Defense Secretary Ashton Carter has apologized for the Army's secret shipment of anthrax samples to close ally South Korea and possibly to Australia.

Carter pledged to hold accountable those responsible for what the Pentagon called the "inadvertent" shipments from the Army's Dugway Proving Ground in Utah to at least 24 labs in the U.S., one in South Korea and one in Australia.

Carter made the apology Saturday to his South Korean counterpart in a bilateral meeting in Singapore on the sidelines of the annual Asian security summit known as the Shangri-La Dialogue.

"Carter extended his apology to South Korean Defense Minister Han Min-koo for the incident and vowed to take appropriate measures to prevent a recurrence," the South Korean Defense Ministry confirmed in a statement.

Dugway sent an anthrax sample, which had been irradiated and was supposed to have been inert, to the Osan Air Force Base without the knowledge of the Seoul government.

Under the U.S.-South Korea Status of Forces agreement, the U.S. was not obligated to notify South Korea of the shipment because the anthrax sample was believed to be inert, U.S. Forces Korea (USFK) said in a statement.

The anthrax sample was intended to be used for training to "defend the Korean people by testing currently fielded equipment and new systems that could better identify toxins and pathogens in the environment," USFK said.

The sample at Osan has since been destroyed. As a precaution, 22 military and civilian personnel at Osan have been treated with antibiotics for possible exposure to the deadly toxin. USFK has stressed that there is no threat to the general public.

In an editorial, the Chosun Ilbo, South Korea's largest circulation newspaper, said the U.S. was justified in training against biological agents but "the problem is that the South Korean government was oblivious to the entry of such a deadly substance."

"The bacteria can be transmitted by air or through food," the editorial said. "Anthrax spores are strong enough to survive for more than 10 years in the soil. That is why there is a ban on the transport of samples. But the U.S. military failed to abide by the basic rules."

The Pentagon has also said that live anthrax from the same Dugway batch may have been sent to Australia in a sample that was shipped to an as yet unidentified lab in Australia in 2008.

On Saturday, Australian Foreign Minister Julie Bishop said: "We're assured by the U.S. military there is no risk to public health. We're taking part in the investigation and I'm pleased it is under way."

However, Nick Xenophon, an independent Australian senator, said: "The fact that this occurred some seven years ago and we're only hearing about it now -- it could be the cover-up is almost as bad as the incident itself," the Australian Associated Press reported.

Last week, Pentagon officials initially announced that supposedly inert anthrax samples from Dugway had been shipped to labs in nine states, but a sample sent to an unidentified lab in Maryland had turned up live.

Four personnel at the civilian lab in Maryland were treated with antibiotics as a precaution.

The Centers for Disease Control and Prevention (CDC) in Atlanta was called in to lead an investigation.

The Pentagon later said that samples from the same batch at Dugway had gone to 24 labs in 11 states, and also to South Korea and Australia.

On Friday, Deputy Defense Secretary Robert Work ordered a "comprehensive review of DoD laboratory procedures, processes, and protocols associated with inactivating spore-forming anthrax."

The samples from Dugway were sent by commercial shipment, including some by Federal Express, USA Today reported.

In a statement, Federal Express said: "We can confirm that FedEx transported shipments for the Department of Defense from the Dugway Proving Ground in Utah,"

"All shipments have been safely delivered to their destinations without incident," the statement said. "We are working closely with the Centers for Disease Control and Prevention to ensure appropriate shipping protocols, policies and regulations are followed by the (Pentagon) on future shipments."

-- Richard Sisk can be reached at Richard.Sisk@military.com