Japan's top government spokesperson says Japan has protested to China over a military ship's intrusion into Japanese waters, and will keep a close watch on the moves of Chinese forces.

Chief Cabinet Secretary Hayashi Yoshimasa said on Monday that Japan has lodged a protest with Beijing over a Chinese survey vessel's entry into Japanese waters off Kagoshima Prefecture on Saturday.

He said Japan made the protest within the day through diplomatic channels and expressed strong concern.

Hayashi said the protest was also based on a Chinese military aircraft's intrusion into Japan's territorial airspace over waters off Nagasaki Prefecture on August 26 as well as the recent moves of Chinese naval vessels.

Hayashi said it would be difficult to assess with certainty the purpose and intention of the Chinese survey ship's latest move in connection with the airspace violation. But he added that China has been increasing its military activities around Japan in recent years.

He said Japan will continue to keep a close watch on China's moves with strong concern, and will do its utmost in its vigilance and surveillance activities.