At least 22 personnel at an US airbase in South Korea could have been exposed to live anthrax packages inadvertently sent by American forces.
Four defence department workers in the US are also undergoing post-exposure treatment.
The US Department of Defense lab in Dugway, Utah, had admitted shipping live anthrax samples mistakenly to several locations. The Pentagon said the packages containing leftover samples were sent to at least nine states in the US.
"Twenty-two personnel may have been exposed during the training event and all personnel were provided appropriate precautionary medical measures to include examinations, antibiotics and in some instances, vaccinations," a statement from the Osan Air Force base said. The airbase is located about 100kms from the South Korean capital Seoul.
The anthrax samples have been destroyed and the facility has been decontaminated.
The samples were thought to have expired and hence less rigorous precautions were observed than handling live agents. An investigation is under way to find out what went wrong.
The Pentagon said the public is not at risk from the leak.
"There is no known risk to the general public, and there are no suspected or confirmed cases of anthrax infection in potentially exposed lab workers," said spokesperson Colonel Steve Warren.