The P-3CK four-engine anti-submarine and maritime surveillance plane dropped three Harpoon missiles, a torpedo and depth charges, Yonhap news agency reported, citing the South Korean military.
The weapons were not armed. The weapons fell into the water about 50 kilometers (31 miles) to the east of Yangyang and are now being retrieved.
A fishing boat was reportedly in the area but was unaffected.
The incident coincides with a New Year’s statement from North Korean leader Kim Jong Un saying that North Korea is developing prohibited intercontinental ballistic missiles. Last year, North Korea announced it was capable of launching a nuclear attack.