Japan plans to study how it could expand coral reefs around a pair of Pacific Ocean outcroppings at the center of a territorial dispute with China, the government said Wednesday.

The three-year research plan is the latest in a series of steps by Japan to fortify the tiny islets, which it uses as the basis for extending its exclusive economic zone, which is larger than Japan itself, far into the Pacific.

The Fisheries Agency is asking for 400 million yen in next year's budget so it can scrutinize the coral at the Okinotorishima islets, an agency official said.

Ryutaro Yamamoto, an agency official, said it decided to conduct research following reports that the coral surrounding the islets is declining.

"Since this is Japan's territory, we have to take some measures as conditions deteriorate," Yamamoto said.

Japan claims the Okinotorishima outcroppings, which are heavily fortified by cement embankments and barely above water at high tide, are islands that enable it to extend its exclusive economic zone.

However, China argues that they are only rocks, not islands governed by rules of sovereignty. It says the reef therefore can't be used by Japan to extend its economic zone and prevent Beijing from exploiting natural resources in the surrounding Pacific Ocean area.

The dispute heated up last year after a Chinese research ship twice came within 200 nautical miles of the crags, triggering a diplomatic protest by Japan. China said the ship was conducting scientific exploration on the "high seas."