Two South Korean air force planes collided in mid-air during training on Friday and Crashed into a mountain in the southeastern city of Sacheon, killing three people and injuring one, emergency officials said.

The officials, speaking on condition of anonymity citing department rules, said they didn’t receive any reports of civilian casualties or damage on the ground.

Three helicopters, 20 vehicles, and dozens of emergency workers were dispatched to the scene, officials said.

The air force confirmed the collision of two KT-1 trainer aircraft. But an air force statement said it was trying to confirm the reported casualties and whether pilots on the aircraft tried to eject to safety. It said a KT-1 aircraft is a two-seat plane.

The cause of the collision wasn’t immediately known.

Friday’s incident came months after a South Korean air force pilot died in January after his F-5E fighter jet crashed into a mountain in the city of Hwaseong, south of the capital, Seoul.

South Korea has around 560,000 troops under a conscription system meant to help deter potential aggression from North Korea, which has about 1.3 million soldiers, one of the largest militaries in the world. About 28,500 American troops are stationed in South Korea, a legacy of the 1950-53 Korean War that ended with an armistice, not a peace treaty.