Human error probably was not a factor in the US Army's mistaken shipment of live anthrax samples to numerous US government and commercial laboratories in the United States and in South Korea, the Army's top general has said.

General Ray Odierno, the US Army chief of staff, told reporters the problem may have been a failure in the technical process of killing, or inactivating, anthrax samples. The process in this case "might not have completely killed" the samples as intended before they were shipped, he said.

US officials at Osan Air Base in South Korea said that the anthrax bacteria it received for training purposes "might not be an inert training sample as expected", and as a result, the suspect anthrax was destroyed by hazardous materials teams.

They said 22 people at Osan "may have been exposed" to the live anthrax and were given precautionary medical measures, including examinations, antibiotics and vaccinations.

"None of the personnel have shown any signs of possible exposure," the air base said in a written statement.

Gen Odierno said the Centres for Disease Control and Prevention is investigating that aspect of what went wrong at Dugway Proving Ground, the Army installation in the state of Utah that sent the anthrax to government and commercial labs in nine states across the US and to a lab in South Korea.

The general said he was not aware that such a problem had surfaced previously at Dugway.

"The best I can tell, it was not human error," he said, adding that normal procedures had been followed.

The Pentagon disclosed yesterday that at least one of nine labs in the US that received anthrax from Dugway got live rather than dead bacteria. It has not identified any of the US laboratories by name.