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 told a regular news conference, adding that Japan would "decisively protect" its territories.

The Russian Defense Ministry said Russian aircraft, including a Tu-95MS nuclear weapon-mountable, long-range strategic bomber, have conducted "patrol flights" with Chinese H-6 bombers above the Sea of Japan and the East China Sea. China's Defense Ministry also said its aircraft conducted the flights with Russian airplanes.

Japan's Defense Ministry said its Air Self-Defense Force scrambled the fighter jets in response.

The ministry said the two Chinese H-6 bombers flew through the Tsushima Strait in southwestern Japan from the East China Sea to the Sea of Japan in the morning before heading northward by passing west of the islets of Takeshima.

Almost at the same time, the two suspected Russian aircraft, which were flying southward above waters north of Takeshima, reversed their direction and headed for the north, according to the ministry.

The four planes did not enter Japan's territorial airspace, the ministry added.

On May 24, China and Russia flew six strategic bombers over waters near Japan, according to the ministry, in what the Japanese defense minister at the time believed was a protest against a "Quad" summit held the same day in Tokyo involving Japan, the United States, Australia and India.