South Korea's military has accused Japan of a "clear provocation" over what it says was a threatening low-altitude flight by a Japanese patrol plane over a South Korean warship.
South Korea says the plane flew 60 to 70 metres above the water near a South Korean naval ship.
Seoul and Tokyo are key US allies and close economic partners, but are currently locked in disputes over military issues and wartime history.
South Korea also accused another Japanese patrol plane of conducting a low-altitude flight over a South Korean warship that had been rescuing a North Korean fishing boat in December.
Japan has claimed the warship locked its fire-control radar onto the plane, but Seoul denied that.
