A Chinese naval ship chased a Russian warship on Monday just outside Japanese territorial waters around the Senkaku Islands in the East China Sea, the government said, in an apparent attempt to demonstrate to others that Beijing has sovereignty over the Tokyo-controlled islets.

A Chinese frigate sailed in the so-called contiguous zone of the uninhabited islets, which Beijing claims and calls Diaoyu, for about six minutes from 7:44 a.m., chasing a Russian frigate after the Russian ship entered the waters, according to the Defense Ministry.

The Russian vessel stayed in the waters from 7:05 a.m. to 8:16 a.m. and appears to have done so to avoid a typhoon, a ministry official said.

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)

By making it appear as if it were patrolling around its own territory, China was likely trying to demonstrate its territorial claim to the Senkakus, according to the official, who added that the move "unilaterally heightens tensions" and has created a "seriously concerning situation."

After the incident, coming amid China's growing maritime assertiveness and increasingly robust China-Russia military ties, Tokyo lodged a protest with Beijing, Deputy Chief Cabinet Secretary Seiji Kihara said at a press conference.

Japan will "respond firmly but in a calm manner" against similar moves, Kihara 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.

Japanese Senior Deputy Foreign Minister Shigeo Yamada protested to Chinese Ambassador to Japan Kong Xuanyou by phone, expressing "grave concern" over the incident, the Foreign Ministry said.

Through a diplomatic channel, Tokyo also called on Moscow to take an "appropriate response," while expressing its "keen interest" in Russia's recent moves around the Senkakus, according to a ministry source.

But Japan stopped short of protesting to Russia given that the country does not lay claims to the Senkakus, as well as that the Russian frigate entered the contiguous zone to avoid the typhoon, the source added.

The ministry also said there are no reports of the Chinese and Russian frigates having entered the territorial waters around the islet group.

Under international law, ships of any nation, including warships, are understood to have the right to sail through the contiguous waters of a coastal nation unless they threaten the nation's safety.

Japan has over the years lodged protests to China over repeated intrusions by Chinese coast guard ships into Japanese waters around the Senkaku Islands.

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