Japan has lodged a protest with China after three Chinese patrol ships entered waters near a set of disputed islands in the East China Sea.

Japan's Chief Cabinet Secretary Osamu Fujimura told reporters Wednesday that the islands are “inherently” Japanese territory.

China, which also claims ownership, says the three vessels were fishery patrol ships conducting routine sweeps of the East China Sea.

The uninhabited islands known as Senkaku in Japanese and Diaoyo in Chinese are a frequent flashpoint between Tokyo and Beijing. They are located in a gas-rich area and surrounded by rich fishing grounds.

Japan's prime minister said Saturday his government is considering buying the privately owned islands.