The helicopter, believed to be an AH-64 Apache, crashed in the city of Wonju, about 70 kilometres (42 miles) southeast of Seoul, a US military spokesman told AFP.
A firefighter in Wonju said the crash took place at 6:22 pm (local time) near the house of the resident who first reported the accident to police.
"The helicopter was burnt to ashes after catching fire," he said, adding two pilots died at the scene.
The firefighter said the exact cause of the crash was unknown.