The remarks came after Japanese Chief Cabinet Secretary Osamu Fujimura said Monday that Japan has registered one of the four islands near the Diaoyu Islands as a national asset.

"Any unilateral actions adopted by the Japanese side concerning the Diaoyu Islands and their affiliated isles are illegal and invalid," Chinese Foreign Ministry spokesman Hong Lei said at a routine press briefing.

China holds an indisputable sovereignty over them, Hong said, stressing "China will continue to take necessary measures to firmly safeguard sovereignty over the Diaoyu Islands and the affiliated isles".