Chinese and Russian military officials are likely to have agreed to strengthen their surveillance activities to boost "strategic deterrence against Japan," sources close to the matter said Thursday, reflecting Beijing and Moscow's relations with Tokyo deteriorating recently.

But China and Russia have also decided not to carry out joint surveillance around Japan for the time being, the Chinese sources said, adding such agreements were made during an online meeting in June between senior military officials of the two countries.

Undated photo shows a Chinese Navy frigate sailing just outside Japanese territorial waters around the Senkaku Islands in the East China Sea.  (Photo courtesy of the Japanese Defense Ministry)(Kyodo)

Beijing and Moscow are expected to increase military activities in waters surrounding the Japanese archipelago, including the East China Sea, as they have been at odds with Tokyo over Taiwan which China regards as a renegade province to be reunified, and Russia's invasion of Ukraine, foreign affairs experts said.

On Monday, a Chinese naval ship chased a Russian warship just outside Japanese territorial waters around the Japan-controlled, China-claimed Senkaku Islands in the East China Sea.

A Chinese frigate sailed in the so-called contiguous zone of the uninhabited islands, which Beijing calls Diaoyu, for about six minutes from 7:44 a.m., chasing a Russian frigate after the nation's ship entered the waters, Japan's Defense Ministry said.

It was the first time since June 2016 that Chinese and Russian naval vessels had been spotted entering the contiguous zone at around the same time.

Over the years, Japan has lodged protests with China over repeated intrusions by Chinese coast guard ships into Japanese waters around the Senkakus.

During the online meeting, China and Russia agreed to notify each other in advance when Russian troops get close to the Senkakus and the Taiwan Strait, according to the sources.

Monday's move by Beijing and Moscow was not a joint action, with the Chinese ship simply chasing the Russian vessel as it had received no advance notice and the Russian frigate is believed to have been near the Senkakus to avoid a typhoon, the sources said.

The agreement on advance notification is also applied when the Chinese military approaches areas near four islands controlled by Moscow off Japan's northernmost main island Hokkaido.

Japan claims the Soviet Union illegally seized the four islands -- Kunashiri, Etorofu, Shikotan and the Habomai group of islets -- soon after Japan's surrender in World War II in August 1945. The islands are called the Northern Territories by Japan and the Southern Kurils by Russia.

Tokyo's relations with Moscow have been also souring as Russia has opposed Japan's economic sanctions on the nation over the Ukraine crisis. Their negotiations for a postwar peace treaty have been suspended in recent months.

Japan has been wary of the deepening military cooperation between China and Russia of late, such as a joint flight of bombers of the two countries over the Sea of Japan, the East China Sea and the Pacific in late May.