They first rescued two pilots and later found a mechanic, who was only identified by the surname Yoon, suffering apparent cardiac arrest.

The authorities said the three were taken to a nearby hospital where the mechanic was pronounced dead.

The Kamov KA-32 helicopter belongs to the Korea Forest Service and was reported departing from Gimpo Airport to put out a fire on Mount Youngchuk in eastern Seoul. A KFS official said the helicopter apparently fell while it tried to take water from the river. The fire on the mountain was extinguished.

One witness said that the helicopter suddenly plunged into the water after it was put on standby in the air. The authorities said they will investigate the cause of the accident later.

"Two people are still recovering from their injuries, so it's difficult to get their statements at this moment," South Korean police said. "We need cooperation from other institutions to investigate the cause of the accident or conduct an autopsy on the deceased."

The Kamov KA-32 model was adopted from Russia in 1997 for the main purpose for putting out mountain fires. (Yonhap)