Japan says it has spotted seven Chinese naval ships near one of its islands, as tensions between the two Asian powers remain high over a territorial dispute in the East China Sea.

Japan's Defense Ministry says the Chinese ships were spotted early Tuesday by Japanese aircraft about 50 kilometers outside Yonaguni Island, Japan's westernmost island.

The ministry said the ships were in the contiguous zone, an area just outside Japan's territorial waters in which Japan has certain rights under international law.

China has increasingly sent ships to patrol Japanese-controlled areas since September, when Japan nationalized a group of islands claimed by Beijing. So far there have been no clashes.

The uninhabited islands at the center of the dispute are called Senkaku in Japan and Diaoyu in China. They are surrounded by valuable fishing grounds and potential energy deposits.

Japan says it purchased some the islands from their private Japanese landowners to ensure their stability. But Beijing has responded furiously, saying they are part of their “sacred territory” since ancient times.