Japan has lodged a formal protest after a South Korean opposition party leader made a trip to disputed islets, stoking a long-standing territorial spat ahead of an expected three-way summit with China.

"It is totally unacceptable and extremely regrettable that the South Korean opposition party leader landed on Takeshima despite repeated requests from Japan to refrain," Chief Cabinet Secretary Yoshimasa Hayashi, the government's top spokesman, said Tuesday.

Former South Korean Justice Minister Cho Kuk had on Monday visited Takeshima, a small, rocky group of isles, known as Dokdo by Seoul.

Seoul has controlled the islands off its east coast since 1945, when Tokyo's colonial rule of the Korean Peninsula ended, but Japan says they are being illegally occupied.

Japan "lodged a strong protest against South Korea through diplomatic routes" on Monday, Hayashi said, repeating a previously issued Foreign Ministry statement.

A trilateral summit between Japan, South Korea and China is expected to take place in late May in South Korea, but a date for the talks has not yet been announced.

Cho was tipped to run for president before a 2019 academic admissions scandal engulfed his family.

In elections last month, his newly founded Rebuilding Korea party capitalized on discontent with the two main parties to pick up 12 seats.

"Japan's claim to sovereignty over Dokdo is an assertion that Japan's horrific wartime crimes, including invasion and war, massacres and plunder, torture and imprisonment of independence fighters, forced labor, and mobilization of comfort women, were justified," Cho said on his party's X account on Monday.

In January, Seoul lodged a "stern" protest against Tokyo's inclusion of the same disputed islets in a tsunami advisory issued after a major earthquake.