"We will closely monitor the increasing cooperation between the two countries with a sense of concern," Matsuno, Japan's top government spokesperson, told a regular press conference, adding that Japan would "decisively protect" its territories.

The Russian defence ministry on Wednesday announced its military and Chinese counterpart conducted joint patrols of strategic warplanes over the Sea of Japan and East China Sea.

(Reporting by Kantaro Komiya; Editing by Tom Hogue)