Japan has told China and Russia it has "severe concerns" over their frequent joint air force activities around Japan's territory, Chief Cabinet Secretary Hirokazu Matsuno said on Thursday.

"We will closely monitor the increasing cooperation between the two countries with a sense of concern," Matsuno (above), 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.

- Reuters