Japan has lodged a protest with South Korea over its reported plan to create a new military unit to defend the Seoul-controlled, Tokyo-claimed islets of Takeshima in the Sea of Japan, Foreign Minister Taro Kono said Friday.

South Korea calls the pair of rocks Dokdo and maintains a small police garrison there.

Kono expressed concern that Seoul's move may undermine bilateral relations at a time when close cooperation with Japan and the United States is needed to address North Korea's military provocations.

"We cannot totally accept (the plan). We strongly protested it," Kono told reporters. The sovereignty dispute over the pair of rocky outcroppings remains a source of friction.

"Under the current security situation, Japan, the United States and South Korea have to strengthen ties in a future-oriented manner. (This decision) runs counter to that," Kono said.

On Thursday, local media reported that the South Korean military had announced a plan to set up the new troop unit sometime between 2018 and 2020 to defend "strategic islands" including the disputed islets and Ulleung Island, which lies roughly 90 km northwest.