Four Chinese coastguard ships sailed through disputed waters off Tokyo-controlled islands in the East China Sea Saturday, officials said, as Japan continued a massive drill aimed at bolstering defence of its remote islands. The vessels entered 12-nautical-mile territorial waters at about 9:25 am (0025 GMT) off one of the Senkaku islands, which China also claims and calls the Diaoyus, the Japan Coast Guard said. They left the 12-nautical-mile zone about two hours later, the coastguard said. Nine days ago, four Chinese coastguard ships were also spotted sailing through the waters, according to the Japanese coastguard. China's state-owned ships and aircraft have approached the Senkakus on and off to demonstrate its territorial claims, especially after Japan nationalised some of the islands in September last year. They have been spotted inside the territorial zone on 50 days so far this year, the coastguard said. Those Chinese vessels have been chased by Japanese coastguard ships with the two sides trading warnings and claims of sovereignty. Usually they stay for a few hours and then move out into waters surrounding the territorial zone. Japan's defence forces launched an 18-day drill on November 1 with 34,000 military personnel, six vessels and 360 aircraft, making China nervous. They stationed missiles, albeit not operable, on islands marking the gateway to the Pacific in the exercise. The exercise comes amid growing nervousness in Japan and other parts of Asia over China's surging military might, which has seen it expand its naval reach into the Pacific Ocean.