The pilot of a civilian helicopter, who was the sole occupant, was killed after the chopper crashed into a reservoir in a northern city Tuesday, rescue officials said.

The aircraft, carrying the 67-year-old pilot, crashed into the reservoir in Pocheon, 52 kilometers northeast of Seoul, at 11:08 a.m., according to the officials.

Rescue authorities found the pilot dead inside the submerged aircraft at about 3 p.m. after mobilizing divers and rescue boats for the search operation in the reservoir, estimated to be around 10-13 meters deep.

The operation had experienced difficulty due to the reservoir's depth and murky water.

The AS-350 helicopter crashed during an inspection of its equipment ahead of being deployed Wednesday for the seasonal fire prevention period.

The four-decade-long pilot reportedly headed to Pocheon alone without waiting for his fellow passenger to collect water for its bucket from the reservoir.

The civilian helicopter company said the water collecting test was scheduled from 1-4 p.m.

It is expected to take at least several days for the authorities to recover the sunken helicopter.

Video footage taken by people near the scene showed the helicopter descending a bucket suspended by a cable into the reservoir before the aircraft went down vertically and became partially submerged.

The helicopter then rose into the air before spinning out of control and crashing into the reservoir.