Two Chinook helicopters flew over Chesterfield this morning.
The Chinooks - large, tandem rotor helicopters operated by the Royal Air Force (RAF) - were seen above the town at around 9am.
A Ministry of Defence spokesperson said: "People shouldn't worry - there's no invasion going on.
"They may have been training.
"There's quite a lot of Chinook bases nearby."
According to the RAF's website: "The Chinook is an extremely capable and highly versatile support helicopter that can be operated from land or sea bases into a range of diverse environments, from the Arctic to the desert or jungle. The aircraft may be armed and is fitted with a suite of self-defence equipment allowing it to operate across the battlespace. Chinooks are primarily used for trooping, resupply and battlefield casualty evacuation.
"With its triple-hook external load system, internal cargo winch, roller conveyor fit and large reserves of power, the aircraft can lift a wide variety of complex underslung or internal freight, including vehicles. It can carry up to 55 troops or up to approximately 10 tonnes of mixed cargo.
"Secondary roles include search and rescue and supporting a wide variety of specialist tasks, including the National Resilience commitment. A Chinook crew comprises two pilots and two crewmen, supplemented by specialists dependent upon mission requirements."