China and Russia have agreed to deepen strategic coordination in the defense sector triggering concern in Tokyo.

According to Anadolu Agency, repeated military exercises by China and Russia can be regarded as “demonstrative action toward Japan,” said Japan’s Defense Minister Kishi Nobuo.

“The possibility cannot be ruled out that they will deepen their military coordination,” he added, according to a report by NHK News on Wednesday.

The concern raised by Japan came as the defense ministers of China and Russia said “the solidarity between China and Russia is as strong as a mountain and friendship is unbreakable.”

Chinese Minister of National Defense Gen. Wei Fenghe held a video call with his Russian counterpart Gen. Sergei Shoigu on Tuesday.

“The two militaries will work together to comprehensively implement the important consensus reached by the heads of state of the two countries, continue to deepen strategic coordination and strengthen cooperation in strategic exercises and joint patrols, so as to make new contributions to safeguarding the core interests of the two countries and protecting international and regional security and stability,” according to a statement released by Chinese Defense Ministry.

Japan’s Kishi in his statement was referring to joint flights earlier this month by Chinese and Russian bombers near Japan.

The country’s Defense Ministry said “two aircraft from each country flew over the Sea of Japan and the East China Sea before heading toward the Pacific Ocean on Friday.”

It was the third time since December last year that Japan's Defense Ministry has made public information about Chinese and Russian joint flights.

Expressing grave concern, Kishi said these moves clearly “indicate the security situation surrounding Japan is growing more severe.”

Last month, navies of Russia and China also held joint naval exercises in the Sea of Japan.

In their joint patrols, the two navies held their first joint passage through the Tsugaru Strait in the Japanese archipelago.

Following the joint exercises by China and Russia, the navies of the US and Japan also held military exercises in contested waters of the South China Sea.