TOKYO - A survey group organized by Tokyo metropolitan government arrived at waters near Diaoyu Islands in the East China Sea and started illegal survey around the islands Sunday morning, local media reported.

The group departed from Ishigaki port, Okinawa, Saturday night and arrived at waters surrounding Diaoyu Islands at 5 am Sunday local time. The group started illegal survey at 6:15 am, including measuring the temperature and the depth of the water and the landform of the area.

The city government of Tokyo's survey staff approach Diaoyu islands in the East China Sea in this photo by Kyodo Sept 2, 2012. [Photo/Agencies]

The group is expected to return to Ishigaki port Sunday night, according to media reports.

Tokyo metropolitan government said it plans to study the natural environment of the Diaoyu Islands because it's a necessary step for buying the islands.

The investigation group comprises a total of 25 people, including officials from Tokyo and Ishigaki governments, estate valuers and a university professor.

Tokyo government earlier sent a landing application, but was rejected by the central government Monday for security reasons.

Tokyo is also preparing to hold another survey on the Diaoyu Islands in October in which the governor of Tokyo Shintaro Ishihara will take part.

Ishihara has announced in April that the Tokyo government plans to purchase three of the five?uninhabited islands.

China has been repeatedly reiterating that the Diaoyu Islands and their adjacent islets have been inherent parts of Chinese territory since ancient times and China has indisputable historical and jurisprudential evidence for this, and any unilateral action taken by Japan on the Diaoyu Islands is illegal and invalid. China urges Japan to handle the recent problems properly to avoid any harm of the China-Japan ties.