Japan has lodged protests with South Korea over military drills by the latter on disputed islands, calling the move "extremely regrettable."

Japan's Foreign Ministry in a statement Friday said it summoned a senior diplomat at the South Korean Embassy in Tokyo, while the Japanese Embassy in Seoul also summoned a senior South Korean official to make the protest.

"Takeshima is indisputably an inherent territory of Japan, in light of historical facts and based on international law," the ministry said in the statement. "The drills by the South Korean military are unacceptable and extremely regrettable."

The two nations have long been at loggerheads over the sovereignty of the group of islets called Takeshima in Japan and Dokdo in Korea, which lie about halfway between the East Asian neighbors in the Sea of Japan, which South Korea calls the East Sea.

The Japanese protest comes even as bilateral frictions have eased recently amid shared concerns about China's growing might and U.S. entreaties for its allies to work more closely together. U.S., Japanese and South Korean leaders have been expected to hold a trilateral summit in the United States this summer.

Asked about the Japanese protest, a spokesperson for the U.S. State Department said the question of sovereignty was for South Korea and Japan to resolve, but added that "a robust and effective trilateral relationship ... is critical for our shared security and common interests."

A South Korean military official said his country had conducted the military drills routinely every year.

"The East Sea territory defense exercise was carried out to conduct our mission to protect our territory, people and property," the official said.