The South Korean navy held a rare live-fire drill near a set of disputed islands on Friday, brushing off angry protests from co-claimant Japan, which called the exercises "deplorable".
The defence ministry said the drill around the Seoul-controlled islets, called Dokdo by the South and Takeshima by Japan, were part of the military's "regular" national defence training.
The navy and coastguard have staged joint exercises near Dokdo many times, but a live-fire drill is rare and it prompted an angry response from Japan.
"Japan can never accept the drill given its position on Takeshima, and so we strongly demanded that the South Korean government stop its plans," Chief Cabinet Secretary Yoshihide Suga told reporters in Tokyo.
Suga, the government's top spokesman, said the decision to push ahead with the exercises was "extremely deplorable".
Seoul dismissed the Japanese protests out of hand.
"This is a military drill to bolster the defence of the Republic of Korea, so any outside demand or interference is not a subject for consideration," ministry spokesman Wi Yong-Seop said.
"The drill is now being carried out as scheduled," Wi said.
A total of 19 naval ships and aircraft were involved, including a light destroyer and a Lynx anti-submarine helicopter.
A Navy spokesman said the exercise was playing out defence scenarios against a variety of potential antagonists, "including North Korea".
Earlier this week, North Korean state media released pictures of leader Kim Jong-un overseeing a naval drill from the turret of a submarine.
The Dokdo-Takeshima islands have been the subject of a bitter and decades-old territorial dispute between South Korea and Japan.
The row escalated in 2012 following a surprise Dokdo visit by then South Korean President Lee Myung-Bak.
Relations between the two neighbours are currently at their lowest ebb for years, with Washington viewing the rift between its two main Asian military allies with growing alarm.
The tensions are largely linked to Japan's 1910-45 colonial rule over the Korean peninsula.
Many South Koreans believe Japan has failed to properly atone for abuses carried out during the occupation.
The live-fire drill came as Japan began a controversial review of its landmark 1993 apology over the use of many South Korean and Chinese women as wartime sex slaves for its troops.
Japan is also embroiled in a territorial dispute with China over a separate set of islands in the East China Sea.
The Japanese coastguard said two Chinese coastguard vessels had encroached Friday within the 12-nautical-mile band around one of the Senkaku islands, which China also claims and calls the Diaoyus.