Stressing that there was “no risk” to the public, the statement from Osan Air Force Base said an alarm had been sounded after it was discovered that the bacteria being used in the exercise might not have been an inert training sample.

“Twenty-two personnel may have been exposed during the training event,” the statement said, while adding that none had shown any actual exposure symptoms.

The sample was being used in a “laboratory environment” in a self-contained facility on the base.

“Hazardous material teams immediately cordoned off the facility, decontaminated it … and destroyed the agent,” it said.

Osan Air Base, some 105 kilometres (65 miles) south of Seoul, is home to the 51st Fighter Wing and one of a number of facilities housing the 28,500 US military personnel permanently based in South Korea.