US troops carrying out a military exercise on a decommissioned Bulgarian airfield last month entered the wrong building by mistake.

CCTV footage shows the armed troops walking in and out of the factory as bewildered employees look on.

“Soldiers entered and cleared a building next to the airfield that they believed was part of the training area, but that was occupied by Bulgarian civilians operating a private business,” the US Army said in a statement.

“No weapons were fired at any time during this interaction. The US Army takes training seriously and prioritizes the safety of our soldiers, our allies, and civilians.”

The statement added: “We sincerely apologise to the business and its employees. We always learn from these exercises and are fully investigating the cause of this mistake.

“We will implement rigorous procedures to clearly define our training areas and prevent this type of incident in the future.”