South Korea began two days of expanded military drills on Sunday around a group of small islands also claimed by Japan, prompting a protest from Tokyo amid worsening relations between the two countries.
Tokyo and Seoul have long been at loggerheads over the islands, called Takeshima in Japanese and Dokdo in Korean, which lie about halfway between the East Asian neighbours in the Sea of Japan, also known as the East Sea.
The latest military drills included naval, air, and army forces, and marines, a South Korean ministry of defence official said.
The Japanese foreign ministry called the drills unacceptable and said it had lodged a protest with South Korea calling for them to end.
The islands are "obviously an inherent part of the territory of Japan", Kenji Kanasugi, the director general at the ministry's Asian and Oceanian Affairs Bureau, said in a statement to the South Korean Embassy in Tokyo.
Ko Min-jung, a spokeswoman for South Korea's presidential Blue House, said the drill was an annual exercise and not aimed at any specific country.
"It's an exercise to guard our sovereignty and territory," she said.
The exercise included significantly more South Korean forces than previously involved and spanned a wider area in the sea between South Korea and Japan, a South Korean navy official told Reuters.
For the first time the drills included army special forces and a destroyer equipped with the Aegis missile defence system, the official said, speaking on condition of anonymity.
Tension in the region has spiked amid a worsening political and economic spat between South Korea and Japan, a string of missile launches by North Korea, and increasingly assertive military patrols by China and Russia.
South Korea announced the scrapping of an intelligence-sharing pact with Japan on Thursday, drawing a swift protest from Tokyo and deepening a decades-old dispute over wartime history that has hit trade and undercut security co-operation over North Korea.
Relations between South Korea and Japan began to deteriorate late last year after a diplomatic row over compensation for wartime forced labour during Japan's occupation of Korea.
They soured further when Japan tightened its curbs on exports of high-tech materials needed by South Korea's chip industry, and again this month when Tokyo said it would remove South Korea's fast-track export status.
The disputed islands have long been one of the most sensitive areas of contention between Japan and South Korea.
A detachment of South Korean guards has been stationed there since the 1950s and the country has conducted annual defence drills in the area.
The current exercises had been delayed as relations deteriorated, South Korea's Yonhap news agency reported.
In July, South Korea and Japan responded to what they saw as a breach of their air space near the islands by a Russian military plane.
The South Korean navy said the drills were designed to underscore its commitment to defending the broader area.
"The military has changed the name of the drills to 'East Sea Territorial Protection Exercise', reflecting the scale and meaning of the drills to solidify the military's resolve to protect the territory in the East Sea," the South Korean navy said. Previous drills had been called the "Dokdo Defence Exercise".